diff --git a/eslint-plugins/healthlog/__tests__/no-default-zone-literal.test.mjs b/eslint-plugins/healthlog/__tests__/no-default-zone-literal.test.mjs
new file mode 100644
index 000000000..8213cc9b4
--- /dev/null
+++ b/eslint-plugins/healthlog/__tests__/no-default-zone-literal.test.mjs
@@ -0,0 +1,75 @@
+/**
+ * Unit tests for the `healthlog/no-default-zone-literal` rule: the literal is
+ * banned in application source, allowed in `src/lib/tz/` and in tests, and
+ * the pending files are exempt until their owner switches them.
+ */
+import { describe, it } from "vitest";
+import { RuleTester } from "eslint";
+import rule from "../no-default-zone-literal.js";
+
+RuleTester.describe = describe;
+RuleTester.it = it;
+RuleTester.itOnly = it.only;
+
+const ruleTester = new RuleTester({
+  languageOptions: {
+    ecmaVersion: 2023,
+    sourceType: "module",
+    parserOptions: { ecmaFeatures: { jsx: true } },
+  },
+});
+
+ruleTester.run("no-default-zone-literal", rule, {
+  valid: [
+    {
+      code: 'import { DEFAULT_TIMEZONE } from "@/lib/tz/format"; const tz = user.timezone || DEFAULT_TIMEZONE;',
+      filename: "/repo/src/app/api/insights/cards/route.ts",
+    },
+    {
+      // The one definition.
+      code: 'export const DEFAULT_TIMEZONE = "Europe/Berlin";',
+      filename: "/repo/src/lib/tz/format.ts",
+    },
+    {
+      // Fixtures pick a concrete zone on purpose.
+      code: 'const TZ = "Europe/Berlin";',
+      filename: "/repo/src/lib/insights/__tests__/graded-series.test.ts",
+    },
+    {
+      // Comments are not checked.
+      code: "// read on Europe/Berlin days before v1.39.3\nconst x = 1;",
+      filename: "/repo/src/lib/insights/graded-series.ts",
+    },
+    {
+      // Pending: owned by another change, switched at merge.
+      code: 'const DEFAULT_TZ = "Europe/Berlin";',
+      filename: "/repo/src/lib/medications/window-status.ts",
+    },
+    {
+      code: 'const other = "America/New_York";',
+      filename: "/repo/src/lib/insights/graded-series.ts",
+    },
+  ],
+  invalid: [
+    {
+      code: 'const tz = user.timezone || "Europe/Berlin";',
+      filename: "/repo/src/app/api/insights/cards/route.ts",
+      errors: [{ messageId: "zoneLiteral" }],
+    },
+    {
+      code: 'readDayAggregates({ timeZone: "Europe/Berlin" });',
+      filename: "/repo/src/lib/insights/graded-series.ts",
+      errors: [{ messageId: "zoneLiteral" }],
+    },
+    {
+      code: "const label = `Date: ${todayKey} (Europe/Berlin)`;",
+      filename: "/repo/src/lib/ai/prompts/pulse.ts",
+      errors: [{ messageId: "zoneLiteral" }],
+    },
+    {
+      code: '<input placeholder="Europe/Berlin" />',
+      filename: "/repo/src/components/settings/timezone-picker.tsx",
+      errors: [{ messageId: "zoneLiteral" }],
+    },
+  ],
+});
diff --git a/eslint-plugins/healthlog/index.js b/eslint-plugins/healthlog/index.js
index 8147b1540..961372c8b 100644
--- a/eslint-plugins/healthlog/index.js
+++ b/eslint-plugins/healthlog/index.js
@@ -10,6 +10,8 @@
  *                              and no off-scale `5` step on a bg-card shell.
  *   - `job-handler-outcome`  — pg-boss bindings route through createAndWork,
  *                              whose handler returns a JobOutcome.
+ *   - `no-default-zone-literal` — the default zone is DEFAULT_TIMEZONE from
+ *                              src/lib/tz/format, never written out.
  */
 
 "use strict";
@@ -20,6 +22,7 @@ const apiFetchRequired = require("./api-fetch-required.js");
 const noRawPaletteColor = require("./no-raw-palette-color.js");
 const spacingScale = require("./spacing-scale.js");
 const jobHandlerOutcome = require("./job-handler-outcome.js");
+const noDefaultZoneLiteral = require("./no-default-zone-literal.js");
 
 module.exports = {
   rules: {
@@ -34,5 +37,6 @@ module.exports = {
     // the semantic sweep). See the rule header.
     "no-dracula-utility": noRawPaletteColor,
     "job-handler-outcome": jobHandlerOutcome,
+    "no-default-zone-literal": noDefaultZoneLiteral,
   },
 };
diff --git a/eslint-plugins/healthlog/no-default-zone-literal.js b/eslint-plugins/healthlog/no-default-zone-literal.js
new file mode 100644
index 000000000..f3a26edc3
--- /dev/null
+++ b/eslint-plugins/healthlog/no-default-zone-literal.js
@@ -0,0 +1,102 @@
+/**
+ * @fileoverview ESLint rule — the default zone is named in one place.
+ *
+ * `"Europe/Berlin"` was written out in some eighty places: fallbacks for a
+ * user with no zone, day keys cut in Berlin whatever the user's zone, a
+ * graded series folded on Berlin days, a prompt that labelled every date as
+ * Berlin's. Each copy was a place where a user outside that zone got
+ * someone else's calendar. `DEFAULT_TIMEZONE` in `src/lib/tz/format.ts` is
+ * the one definition; everything else imports it, and a surface that means
+ * the user's own zone asks for it (`resolveUserTimezone`).
+ *
+ * The rule flags a string literal or template element containing
+ * `Europe/Berlin` in `src/`, outside `src/lib/tz/` and outside test files
+ * (fixtures pick concrete zones on purpose). Comments are not checked.
+ *
+ * WHAT THIS RULE DOES NOT CATCH
+ *
+ * A zone assembled from parts (`"Europe/" + city`), a different hard-coded
+ * zone, or a Berlin day computed through an `Intl.DateTimeFormat` built from
+ * a variable. It removes the literal, not every way to hard-code a calendar.
+ *
+ * `PENDING` lists files another change owns at the moment; they switch to the
+ * constant when that change lands, and the list shrinks to nothing.
+ */
+
+"use strict";
+
+const ZONE = "Europe/Berlin";
+
+const EXEMPT_ROOTS = ["src/lib/tz/"];
+
+// Owned by the medication-scheduling change in flight; switched at merge.
+const PENDING = [
+  "src/lib/medications/",
+  "src/lib/jobs/measurement-reminder.ts",
+  "src/lib/measurement-reminders/",
+  "src/app/api/measurement-reminders/",
+];
+
+function toPosix(filename) {
+  return filename.replace(/\\/g, "/");
+}
+
+function isTestFile(posix) {
+  return (
+    /\.test\.[cm]?[jt]sx?$/.test(posix) ||
+    /\.spec\.[cm]?[jt]sx?$/.test(posix) ||
+    posix.includes("/__tests__/") ||
+    posix.includes("/__mocks__/")
+  );
+}
+
+function isEnforced(filename) {
+  const posix = toPosix(filename);
+  if (!posix.includes("/src/") && !posix.startsWith("src/")) return false;
+  if (EXEMPT_ROOTS.some((root) => posix.includes(root))) return false;
+  if (PENDING.some((p) => posix.includes(p))) return false;
+  if (isTestFile(posix)) return false;
+  return true;
+}
+
+/** @type {import("eslint").Rule.RuleModule} */
+const noDefaultZoneLiteralRule = {
+  meta: {
+    type: "problem",
+    docs: {
+      description:
+        "Import DEFAULT_TIMEZONE from @/lib/tz/format instead of writing the zone out.",
+    },
+    schema: [],
+    messages: {
+      zoneLiteral:
+        'Do not write "Europe/Berlin" out. Import DEFAULT_TIMEZONE from "@/lib/tz/format" for a fallback, or use the user\'s own zone (resolveUserTimezone / the profile timezone) for a day boundary.',
+    },
+  },
+  create(context) {
+    const filename = context.filename ?? context.getFilename?.();
+    if (!filename || !isEnforced(filename)) {
+      return {};
+    }
+    return {
+      Literal(node) {
+        if (typeof node.value === "string" && node.value.includes(ZONE)) {
+          context.report({ node, messageId: "zoneLiteral" });
+        }
+      },
+      TemplateElement(node) {
+        const raw = node.value && (node.value.cooked ?? node.value.raw);
+        if (typeof raw === "string" && raw.includes(ZONE)) {
+          context.report({ node, messageId: "zoneLiteral" });
+        }
+      },
+      JSXText(node) {
+        if (typeof node.value === "string" && node.value.includes(ZONE)) {
+          context.report({ node, messageId: "zoneLiteral" });
+        }
+      },
+    };
+  },
+};
+
+module.exports = noDefaultZoneLiteralRule;
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 36fb7684e..1399d6eee 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -101,6 +101,10 @@ const eslintConfig = defineConfig([
       // catches the omission only — a handler that returns `{ ok: true }`
       // after swallowing still passes, and its header says so.
       "healthlog/job-handler-outcome": "error",
+      // The default zone is named once, in src/lib/tz/format.ts. A copy of the
+      // literal anywhere else is where a user outside that zone got someone
+      // else's calendar. src/lib/tz/ and test files are exempt; see the rule.
+      "healthlog/no-default-zone-literal": "error",
     },
   },
   // v1.28.17 — every recharts-rendering component funnels through the
diff --git a/src/__tests__/date-order-explicit-formatter-guard.test.ts b/src/__tests__/date-order-explicit-formatter-guard.test.ts
index faec9324c..cf89630c0 100644
--- a/src/__tests__/date-order-explicit-formatter-guard.test.ts
+++ b/src/__tests__/date-order-explicit-formatter-guard.test.ts
@@ -259,18 +259,10 @@ const RAW_ALLOWLIST: ReadonlyArray<{ file: string; why: string }> = [
     file: "src/components/measurement-reminders/vorsorge-section.tsx",
     why: "The ISO day the postpone request writes to the API. A wire value, and the wire format is ISO whatever the reader prefers.",
   },
-  {
-    file: "src/lib/gamification/achievements.ts",
-    why: "The day key streak arithmetic counts on.",
-  },
   {
     file: "src/lib/charts/bucket-time-series.ts",
     why: "Bucket boundary arithmetic. The bucket LABELS are a separate step and go through `makeBucketLabelFormatters`.",
   },
-  {
-    file: "src/lib/ai/prompts/insight-generator.ts",
-    why: "An ISO day inside a model prompt. The model reads ISO; the user never sees this string.",
-  },
   {
     file: "src/lib/jobs/measurement-reminder.ts",
     why: "The local day in the notification dedup key. Changing it with a display preference would re-send a claimed reminder.",
@@ -283,10 +275,6 @@ const RAW_ALLOWLIST: ReadonlyArray<{ file: string; why: string }> = [
     file: "src/lib/i18n/relative-time.ts",
     why: "The en-CA day keys that decide today / yesterday. A comparison between two keys, never printed.",
   },
-  {
-    file: "src/lib/tz/resolver.ts",
-    why: "The Berlin day formatter the zone arithmetic itself is built on.",
-  },
 ];
 
 /**
diff --git a/src/app/api/admin/notifications/reminder-check/route.ts b/src/app/api/admin/notifications/reminder-check/route.ts
index 6cb32ae31..78b6fc9c8 100644
--- a/src/app/api/admin/notifications/reminder-check/route.ts
+++ b/src/app/api/admin/notifications/reminder-check/route.ts
@@ -9,6 +9,7 @@ import { parseScheduleRecurrence } from "@/lib/medication-schedule";
 import { dispatchLocalisedNotification } from "@/lib/notifications/dispatch-localised";
 import { getUserTodayBounds, getDayOfWeekInTz } from "@/lib/tz/local-day";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const dynamic = "force-dynamic";
 
@@ -116,7 +117,7 @@ export const POST = apiHandler(async (request: NextRequest) => {
   let notificationsSent = 0;
 
   for (const med of medications) {
-    const userTz = med.user.timezone || "Europe/Berlin";
+    const userTz = med.user.timezone || DEFAULT_TIMEZONE;
     const { start: todayStart, end: todayEnd } = getUserTodayBounds(
       now,
       userTz,
diff --git a/src/app/api/coach/reminder-suggestions/route.ts b/src/app/api/coach/reminder-suggestions/route.ts
index de7a7f1ed..eaafd4844 100644
--- a/src/app/api/coach/reminder-suggestions/route.ts
+++ b/src/app/api/coach/reminder-suggestions/route.ts
@@ -44,8 +44,7 @@ import {
   type ReminderScheduleInput,
 } from "@/lib/measurement-reminders/scheduling";
 import { toMeasurementReminderDto } from "@/lib/measurement-reminders/dto";
-
-const DEFAULT_TIMEZONE = "Europe/Berlin";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 async function postAction(request: NextRequest): Promise<Response> {
   const { user } = await requireAuth();
diff --git a/src/app/api/coach/suggested-actions/route.ts b/src/app/api/coach/suggested-actions/route.ts
index 8302bc33c..aa8e73959 100644
--- a/src/app/api/coach/suggested-actions/route.ts
+++ b/src/app/api/coach/suggested-actions/route.ts
@@ -47,8 +47,8 @@ import {
   type ReminderScheduleInput,
 } from "@/lib/measurement-reminders/scheduling";
 import { toMeasurementReminderDto } from "@/lib/measurement-reminders/dto";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
-const DEFAULT_TIMEZONE = "Europe/Berlin";
 const CHECKUP_NOTIFY_HOUR = 9;
 
 async function postAction(request: NextRequest): Promise<Response> {
diff --git a/src/app/api/cycle/calendar/route.ts b/src/app/api/cycle/calendar/route.ts
index 6e68e71f1..7d3b07dda 100644
--- a/src/app/api/cycle/calendar/route.ts
+++ b/src/app/api/cycle/calendar/route.ts
@@ -39,7 +39,8 @@ import {
 import { resolveCycleVerdict } from "@/lib/cycle/verdict";
 import { addDays, dayDiff } from "@/lib/cycle/day-math";
 import { BBT_WINDOW } from "@/lib/cycle/types";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 import { resolveServerLocale } from "@/lib/i18n/server-locale";
 
diff --git a/src/app/api/cycle/day-logs/[id]/route.ts b/src/app/api/cycle/day-logs/[id]/route.ts
index 211e07eea..4c92fe9a2 100644
--- a/src/app/api/cycle/day-logs/[id]/route.ts
+++ b/src/app/api/cycle/day-logs/[id]/route.ts
@@ -28,7 +28,7 @@ import {
   ensureCycleForBleedingDay,
   removeCycleStartedOn,
 } from "@/lib/cycle/cycle-boundaries";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { encrypt, decrypt } from "@/lib/crypto";
 import { getOrCreateCycleProfile } from "@/lib/cycle/profile";
 import { replaceSymptomLinks } from "@/lib/cycle/day-log-write";
diff --git a/src/app/api/cycle/day-logs/bulk/route.ts b/src/app/api/cycle/day-logs/bulk/route.ts
index 93c56116b..66abea65a 100644
--- a/src/app/api/cycle/day-logs/bulk/route.ts
+++ b/src/app/api/cycle/day-logs/bulk/route.ts
@@ -40,7 +40,7 @@ import {
   type UnstableExternalIdShape,
 } from "@/lib/validations/external-id";
 import { upsertCycleDayLog } from "@/lib/cycle/day-log-write";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 const BATCH_RATE_LIMIT_MAX = 60;
 const BATCH_RATE_LIMIT_WINDOW_MS = 60 * 1000;
diff --git a/src/app/api/cycle/day-logs/route.ts b/src/app/api/cycle/day-logs/route.ts
index cacf927fd..158d85c84 100644
--- a/src/app/api/cycle/day-logs/route.ts
+++ b/src/app/api/cycle/day-logs/route.ts
@@ -40,7 +40,7 @@ import {
 import { upsertCycleDayLog } from "@/lib/cycle/day-log-write";
 import { findOwningCycleId } from "@/lib/cycle/cycle-attribution";
 import { toCycleDayLogDTO, dayLogSymptomInclude } from "@/lib/cycle/dto";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const POST = apiHandler(withIdempotency<[NextRequest]>(postDayLog));
 
diff --git a/src/app/api/cycle/insights/route.ts b/src/app/api/cycle/insights/route.ts
index 287406817..4fc89c5b3 100644
--- a/src/app/api/cycle/insights/route.ts
+++ b/src/app/api/cycle/insights/route.ts
@@ -43,7 +43,8 @@ import {
   type SymptomDay,
 } from "@/lib/cycle/symptom-phase";
 import { addDays } from "@/lib/cycle/day-math";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /** Trailing window the phase-contrast walks (days). */
 const WINDOW_DAYS = 365;
@@ -228,6 +229,7 @@ export const GET = apiHandler(async () => {
     phaseByDay,
     measurements,
     userPriorityJson,
+    timeZone: tz,
   });
   const headline = selectHeadlinePhaseRow(rows);
   // The lagged rows are served with their narrated `interpretation` intact, so
@@ -239,6 +241,7 @@ export const GET = apiHandler(async () => {
     phaseByDay,
     measurements,
     userPriorityJson,
+    timeZone: tz,
     locale,
   });
 
diff --git a/src/app/api/cycle/period/route.ts b/src/app/api/cycle/period/route.ts
index 4fc37c6a8..80b276b3f 100644
--- a/src/app/api/cycle/period/route.ts
+++ b/src/app/api/cycle/period/route.ts
@@ -37,7 +37,7 @@ import {
   toMenstrualCycleDTO,
   dayLogSymptomInclude,
 } from "@/lib/cycle/dto";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { openCycleAt, type MovedAnchors } from "@/lib/cycle/cycle-boundaries";
 
 export const POST = apiHandler(withIdempotency<[NextRequest]>(postPeriod));
diff --git a/src/app/api/illness/episodes/[id]/correlation/route.ts b/src/app/api/illness/episodes/[id]/correlation/route.ts
index c0f647364..abc456568 100644
--- a/src/app/api/illness/episodes/[id]/correlation/route.ts
+++ b/src/app/api/illness/episodes/[id]/correlation/route.ts
@@ -22,7 +22,7 @@ import { apiSuccess, apiError } from "@/lib/api-response";
 import { requireIllnessEnabled } from "@/lib/illness/gate";
 import { computeEpisodeCorrelation } from "@/lib/illness/correlation-read";
 import { notifyIllnessRedFlag } from "@/lib/illness/red-flag-notify";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
diff --git a/src/app/api/illness/episodes/[id]/day-logs/route.ts b/src/app/api/illness/episodes/[id]/day-logs/route.ts
index 2202f338f..56b18f31d 100644
--- a/src/app/api/illness/episodes/[id]/day-logs/route.ts
+++ b/src/app/api/illness/episodes/[id]/day-logs/route.ts
@@ -34,7 +34,7 @@ import {
 } from "@/lib/validations/illness";
 import { upsertIllnessDayLog } from "@/lib/illness/day-log-write";
 import { toIllnessDayLogDTO, dayLogSymptomInclude } from "@/lib/illness/dto";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
diff --git a/src/app/api/import/route.ts b/src/app/api/import/route.ts
index 88ea23e34..e827cc674 100644
--- a/src/app/api/import/route.ts
+++ b/src/app/api/import/route.ts
@@ -37,7 +37,8 @@ import type { MeasurementType } from "@/generated/prisma/client";
 import { emitInsertedMeasurementArrivals } from "@/lib/arrivals/measurement-emit";
 import { maybeEnqueueMorningRefresh } from "@/lib/daily/morning-refresh-trigger";
 import { isP2002, isP2025 } from "@/lib/prisma-errors";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { deriveA1 } from "@/lib/mood/level-a";
 import { zonedWallClockToUtc } from "@/lib/tz/wall-clock";
 
diff --git a/src/app/api/insights/cards/route.ts b/src/app/api/insights/cards/route.ts
index b37ae9652..499171ebe 100644
--- a/src/app/api/insights/cards/route.ts
+++ b/src/app/api/insights/cards/route.ts
@@ -34,6 +34,7 @@ import {
 } from "@/lib/analytics/compliance";
 import type { MeasurementType } from "@/generated/prisma/client";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type Severity = "alert" | "caution" | "info" | "good";
 
@@ -216,7 +217,7 @@ export const GET = apiHandler(async () => {
       arr.push(e);
       eventsByMed.set(e.medicationId, arr);
     }
-    const tz = user.timezone || "Europe/Berlin";
+    const tz = user.timezone || DEFAULT_TIMEZONE;
     for (const med of activeMeds) {
       const events = eventsByMed.get(med.id) ?? [];
       const mapped = events.map((e) => ({
diff --git a/src/app/api/insights/comprehensive/route.ts b/src/app/api/insights/comprehensive/route.ts
index 46d6b9be7..cc55cd882 100644
--- a/src/app/api/insights/comprehensive/route.ts
+++ b/src/app/api/insights/comprehensive/route.ts
@@ -41,7 +41,7 @@ import {
   ensureUserMoodRollupsFresh,
   readMoodDayRollups,
 } from "@/lib/rollups/mood-rollups";
-import { userDayKey } from "@/lib/tz/format";
+import { userDayKey, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
 
 export const dynamic = "force-dynamic";
@@ -125,7 +125,7 @@ export async function buildComprehensiveResponse(user: AuthedUser) {
   // pass in this route (resting-pulse proxy, medication-continuity, BP
   // pairing). Was previously re-derived ad hoc (or hardcoded UTC) at each
   // call site.
-  const userTz = user.timezone || "Europe/Berlin";
+  const userTz = user.timezone || DEFAULT_TIMEZONE;
 
   // Fetch user profile (height + DOB drive BMI + BP targets).
   const dbUser = await prisma.user.findUnique({
@@ -445,7 +445,7 @@ export async function buildComprehensiveResponse(user: AuthedUser) {
     const medicationContext = buildComplianceMedicationContext(
       med,
       lastNonSkippedTakenAt(mapped),
-      user.timezone || "Europe/Berlin",
+      user.timezone || DEFAULT_TIMEZONE,
     );
     const c7 = calculateCompliance(mapped, med.schedules, 7, med.createdAt, {
       medicationContext,
diff --git a/src/app/api/insights/correlations/route.ts b/src/app/api/insights/correlations/route.ts
index eb4e4358e..f93c5262f 100644
--- a/src/app/api/insights/correlations/route.ts
+++ b/src/app/api/insights/correlations/route.ts
@@ -41,6 +41,7 @@ import {
 } from "@/lib/insights/correlation-patterns";
 import { assembleDiscoveryMatrix } from "@/lib/insights/discovery-matrix";
 import { fetchLabDraws } from "@/lib/insights/correlation-channel-series";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const dynamic = "force-dynamic";
 
@@ -115,7 +116,7 @@ async function buildCorrelationsResponse(
     where: { id: userId },
     select: { timezone: true },
   });
-  const tz = profile?.timezone ?? "Europe/Berlin";
+  const tz = profile?.timezone ?? DEFAULT_TIMEZONE;
   const now = new Date();
   const since = new Date(now.getTime() - WINDOW_DAYS * MS_PER_DAY);
 
diff --git a/src/app/api/insights/generate/route.ts b/src/app/api/insights/generate/route.ts
index b5706dd8b..266d77959 100644
--- a/src/app/api/insights/generate/route.ts
+++ b/src/app/api/insights/generate/route.ts
@@ -692,6 +692,7 @@ export const POST = apiHandler((request: NextRequest) =>
       userId,
       dbUser?.displayName ?? null,
       locale,
+      await resolveUserTimezone(userId),
     );
 
     // v1.12.7 (B5) — inject the curated SOURCES block for the metric sections
diff --git a/src/app/api/measurements/route.ts b/src/app/api/measurements/route.ts
index ccc8742d6..ffcaafece 100644
--- a/src/app/api/measurements/route.ts
+++ b/src/app/api/measurements/route.ts
@@ -76,6 +76,7 @@ import type {
   MeasurementSource,
   GlucoseContext,
 } from "@/generated/prisma/client";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const GET = apiHandler(async (request: NextRequest) => {
   const { user } = await requireRecordAuth("read", "measurements");
@@ -192,7 +193,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
     const tz =
       user.timezone && user.timezone.length > 0
         ? user.timezone
-        : "Europe/Berlin";
+        : DEFAULT_TIMEZONE;
     // Resolve `[dayStart, dayEnd)` for the requested calendar day in
     // the user's local zone. v1.4.37 W10 — `localDayWindow` honours
     // DST: the previous shape (`canonicalDailyTimestamp ± 12 h`)
@@ -242,7 +243,7 @@ export const GET = apiHandler(async (request: NextRequest) => {
     const tz =
       user.timezone && user.timezone.length > 0
         ? user.timezone
-        : "Europe/Berlin";
+        : DEFAULT_TIMEZONE;
     // v1.29.6 — DESC + take so a long history returns the MOST RECENT
     // window of raw samples for bucketing. The previous `orderBy: asc`
     // + `take: limit` sliced the OLDEST page of rows before the
diff --git a/src/app/api/medications/[id]/compliance/route.ts b/src/app/api/medications/[id]/compliance/route.ts
index c53eeba65..4831388da 100644
--- a/src/app/api/medications/[id]/compliance/route.ts
+++ b/src/app/api/medications/[id]/compliance/route.ts
@@ -9,6 +9,7 @@ import {
 } from "@/lib/medications/compliance-payload";
 import { cachedSwr, caches, type ServerCache } from "@/lib/cache/server-cache";
 import { checkRateLimit } from "@/lib/rate-limit";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -43,7 +44,7 @@ export const GET = apiHandler(
       return apiError("Too many compliance requests. Please retry later.", 429);
     }
 
-    const userTz = user.timezone || "Europe/Berlin";
+    const userTz = user.timezone || DEFAULT_TIMEZONE;
 
     // Read-through the per-user compliance cache (15 min fresh TTL +
     // SWR window). The ownership guard and the medication read live
diff --git a/src/app/api/medications/[id]/dose-history/route.ts b/src/app/api/medications/[id]/dose-history/route.ts
index f494ed66e..0159323a1 100644
--- a/src/app/api/medications/[id]/dose-history/route.ts
+++ b/src/app/api/medications/[id]/dose-history/route.ts
@@ -41,6 +41,7 @@ import {
 } from "@/lib/medications/scheduling/worker-helpers";
 import { assertMedicationOwnership } from "@/lib/medications/route-guards";
 import { dueSchedules } from "@/lib/medications/intake-tracking";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -120,7 +121,7 @@ export const GET = apiHandler(
     }
 
     const now = new Date();
-    const userTz = user.timezone || "Europe/Berlin";
+    const userTz = user.timezone || DEFAULT_TIMEZONE;
 
     const medication = await prisma.medication.findUnique({
       where: { id },
diff --git a/src/app/api/medications/[id]/efficacy/route.ts b/src/app/api/medications/[id]/efficacy/route.ts
index 5bc75939d..53c54fedd 100644
--- a/src/app/api/medications/[id]/efficacy/route.ts
+++ b/src/app/api/medications/[id]/efficacy/route.ts
@@ -2,6 +2,7 @@ import { apiHandler, requireAuth } from "@/lib/api-handler";
 import { apiSuccess, apiError } from "@/lib/api-response";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { buildMedicationEfficacy } from "@/lib/medications/efficacy/build-efficacy";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -31,7 +32,7 @@ export const GET = apiHandler(
       return apiError("Too many efficacy requests. Please retry later.", 429);
     }
 
-    const userTz = user.timezone || "Europe/Berlin";
+    const userTz = user.timezone || DEFAULT_TIMEZONE;
     const dto = await buildMedicationEfficacy(user.id, id, userTz);
     if (!dto) {
       return apiError("Medication not found", 404);
diff --git a/src/app/api/medications/[id]/route.ts b/src/app/api/medications/[id]/route.ts
index 2d281fbff..1afbbbb26 100644
--- a/src/app/api/medications/[id]/route.ts
+++ b/src/app/api/medications/[id]/route.ts
@@ -46,6 +46,7 @@ import {
   isRecordOnly,
   scheduleWireFields,
 } from "@/lib/medications/intake-tracking";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type RouteParams = { params: Promise<{ id: string }> };
 
@@ -175,7 +176,7 @@ export const GET = apiHandler(
       },
       schedules: liveSchedules,
       now,
-      userTz: user.timezone || "Europe/Berlin",
+      userTz: user.timezone || DEFAULT_TIMEZONE,
       lastIntakeAt: lastIntake?.takenAt ?? null,
       resolvedSlots: resolvedRows.map(toResolvedSlotMark),
       eraStart: latestRevision?.validUntil ?? null,
@@ -603,7 +604,7 @@ export const PUT = apiHandler(
       // `syncVersion` so delta-sync clients drop them. The projector and
       // the reminder worker re-mint the anchors for the new times on their
       // next pass — the route does not pre-create them.
-      const userTz = user.timezone || "Europe/Berlin";
+      const userTz = user.timezone || DEFAULT_TIMEZONE;
       const { start: todayStart } = getUserTodayBounds(new Date(), userTz);
       const tombstoneWhere = {
         userId: user.id,
diff --git a/src/app/api/medications/compliance/route.ts b/src/app/api/medications/compliance/route.ts
index 06836c731..eee5e5f97 100644
--- a/src/app/api/medications/compliance/route.ts
+++ b/src/app/api/medications/compliance/route.ts
@@ -12,6 +12,7 @@ import {
 import { cachedSwr, caches, type ServerCache } from "@/lib/cache/server-cache";
 import { checkRateLimit } from "@/lib/rate-limit";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Batched card-compliance read: one round trip for every medication the
@@ -43,7 +44,7 @@ export const GET = apiHandler(async () => {
     return apiError("Too many compliance requests. Please retry later.", 429);
   }
 
-  const userTz = user.timezone || "Europe/Berlin";
+  const userTz = user.timezone || DEFAULT_TIMEZONE;
 
   // Same ordering as the medications list so the page's card order and
   // this payload walk the same sequence.
diff --git a/src/app/api/mood-entries/[id]/route.ts b/src/app/api/mood-entries/[id]/route.ts
index f6535df55..94cb5f59e 100644
--- a/src/app/api/mood-entries/[id]/route.ts
+++ b/src/app/api/mood-entries/[id]/route.ts
@@ -14,7 +14,8 @@ import { NextRequest } from "next/server";
 import { Prisma } from "@/generated/prisma/client";
 import { apiHandler, requireRecordAuth } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
-import { moodDateKey, DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { deriveA1, shapeLevelA } from "@/lib/mood/level-a";
 import { contextForWire, persistMoodContext } from "@/lib/mood/context";
 import { encryptNote, shapeMoodNote } from "@/lib/crypto/note-cipher";
diff --git a/src/app/api/mood-entries/bulk/route.ts b/src/app/api/mood-entries/bulk/route.ts
index 29bb815d7..3a504fa71 100644
--- a/src/app/api/mood-entries/bulk/route.ts
+++ b/src/app/api/mood-entries/bulk/route.ts
@@ -51,7 +51,8 @@ import {
   UNSTABLE_EXTERNAL_ID_REASON,
   type UnstableExternalIdShape,
 } from "@/lib/validations/external-id";
-import { moodDateKey, DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { levelAForWrite } from "@/lib/mood/level-a";
 import { persistMoodContext } from "@/lib/mood/context";
 import { encryptNote } from "@/lib/crypto/note-cipher";
diff --git a/src/app/api/mood-entries/route.ts b/src/app/api/mood-entries/route.ts
index 96fb20d0a..86b14901a 100644
--- a/src/app/api/mood-entries/route.ts
+++ b/src/app/api/mood-entries/route.ts
@@ -24,7 +24,8 @@ import { apiHandler, requireRecordAuth } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
 import { withIdempotency } from "@/lib/idempotency";
 import { encryptNote, shapeMoodNote } from "@/lib/crypto/note-cipher";
-import { moodDateKey, DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { levelAForWrite, shapeLevelA } from "@/lib/mood/level-a";
 import { contextForWire, persistMoodContext } from "@/lib/mood/context";
 import { listMoodEntriesPage } from "@/lib/mood/list-read";
diff --git a/src/app/api/mood/linked-context/route.ts b/src/app/api/mood/linked-context/route.ts
index c66e0030c..8756d58d0 100644
--- a/src/app/api/mood/linked-context/route.ts
+++ b/src/app/api/mood/linked-context/route.ts
@@ -38,8 +38,7 @@ import { apiSuccess, returnAllZodIssues } from "@/lib/api-response";
 import { annotate } from "@/lib/logging/context";
 import { requireModuleEnabled } from "@/lib/modules/gate";
 import { resolveLinkedDayContext } from "@/lib/mood/linked-context";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
-import { isValidTimezone } from "@/lib/tz/format";
+import { DEFAULT_TIMEZONE, isValidTimezone } from "@/lib/tz/format";
 
 export const dynamic = "force-dynamic";
 
diff --git a/src/app/api/sync/state/route.ts b/src/app/api/sync/state/route.ts
index b45e269a1..4d4083640 100644
--- a/src/app/api/sync/state/route.ts
+++ b/src/app/api/sync/state/route.ts
@@ -42,6 +42,7 @@ import {
   NATIVE_REFRESH_TOKEN_DAYS,
   TOMBSTONE_RETENTION_DAYS,
 } from "@/lib/auth/native-client";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const GET = apiHandler(async (_request: NextRequest) => {
   const { user } = await requireAuth();
@@ -119,7 +120,7 @@ export const GET = apiHandler(async (_request: NextRequest) => {
 
   return apiSuccess({
     userId: user.id,
-    timezone: currentUser?.timezone ?? "Europe/Berlin",
+    timezone: currentUser?.timezone ?? DEFAULT_TIMEZONE,
     lastSyncedAt: lastSyncedAt?.toISOString() ?? null,
     serverNow: serverNow.toISOString(),
     measurements: {
diff --git a/src/app/medications/page-client.tsx b/src/app/medications/page-client.tsx
index 54e39bf6d..e37e45adf 100644
--- a/src/app/medications/page-client.tsx
+++ b/src/app/medications/page-client.tsx
@@ -48,6 +48,7 @@ import { apiGet } from "@/lib/api/api-fetch";
 import { useMedicationComplianceSummaryAll } from "@/lib/queries/use-medication-compliance-summary";
 import { useMedicationListLayout } from "@/lib/queries/use-medication-list-layout";
 import { applyMedicationOrder } from "@/lib/medications/medication-order";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface Schedule {
   id: string;
@@ -397,7 +398,7 @@ export default function MedicationsPageClient() {
   // medications past their band tail stay takeable from the batch. Page
   // order is preserved so the confirm dialog lists medications as rendered.
   const dueMeds = deriveDueMedications(activeMeds, {
-    tz: user?.timezone || "Europe/Berlin",
+    tz: user?.timezone || DEFAULT_TIMEZONE,
     thresholds: thresholds ?? undefined,
     doseStatusById: new Map(
       (complianceRows ?? []).map((row) => [
diff --git a/src/components/charts/medication-compliance-chart.tsx b/src/components/charts/medication-compliance-chart.tsx
index 866759999..fa5aefa0e 100644
--- a/src/components/charts/medication-compliance-chart.tsx
+++ b/src/components/charts/medication-compliance-chart.tsx
@@ -69,6 +69,7 @@ import { useViewportWidth } from "@/hooks/use-viewport-width";
 import { chooseTickInterval } from "@/lib/charts/x-axis-density";
 import { apiGet } from "@/lib/api/api-fetch";
 import { shouldFireDataReady } from "@/lib/charts/data-ready-latch";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface DailyCompliancePoint {
   /** Berlin calendar day, "YYYY-MM-DD". */
@@ -208,7 +209,7 @@ interface MedicationComplianceChartProps {
 
 export function MedicationComplianceChart({
   title,
-  userTimezone = "Europe/Berlin",
+  userTimezone = DEFAULT_TIMEZONE,
   compareBaseline,
   onDataReady,
 }: MedicationComplianceChartProps) {
diff --git a/src/components/dashboard/medication-intake-quick-add.tsx b/src/components/dashboard/medication-intake-quick-add.tsx
index 1476bfb78..52788fd3a 100644
--- a/src/components/dashboard/medication-intake-quick-add.tsx
+++ b/src/components/dashboard/medication-intake-quick-add.tsx
@@ -31,6 +31,7 @@ import {
   intakeToastOptions,
   runUndoIntake,
 } from "@/components/medications/use-medication-intake";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * v1.4.37 W7b — dashboard "Hinzufügen" → "Medikamenteneinnahme" quick-add.
@@ -115,7 +116,7 @@ export function MedicationIntakeQuickAdd({
   // source the cards use); Berlin stays the last-resort fallback so
   // logged-out mounts behave unchanged.
   const { user } = useAuth();
-  const userTz = user?.timezone || "Europe/Berlin";
+  const userTz = user?.timezone || DEFAULT_TIMEZONE;
   // The record this dose lands in, or null in the caller's own.
   const recordName = useActiveRecordName();
 
diff --git a/src/components/insights/intraday-pulse-chart.tsx b/src/components/insights/intraday-pulse-chart.tsx
index 965014e94..09e1ae79a 100644
--- a/src/components/insights/intraday-pulse-chart.tsx
+++ b/src/components/insights/intraday-pulse-chart.tsx
@@ -19,7 +19,7 @@ import { useAuth } from "@/hooks/use-auth";
 import { apiGet } from "@/lib/api/api-fetch";
 import { queryKeys } from "@/lib/query-keys";
 import { useTranslations, useFormatters } from "@/lib/i18n/context";
-import { userDayKey, shiftDateKey } from "@/lib/tz/format";
+import { userDayKey, shiftDateKey, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { TileHeader } from "@/components/insights/tile-header";
 import { Button } from "@/components/ui/button";
 import { QueryErrorCard } from "@/components/ui/query-error-card";
@@ -94,7 +94,7 @@ export function IntradayPulseChart({
   const { isAuthenticated } = useAuth();
   const { t } = useTranslations();
   const fmt = useFormatters();
-  const tz = userTimezone ?? "Europe/Berlin";
+  const tz = userTimezone ?? DEFAULT_TIMEZONE;
   const todayKey = userDayKey(new Date(), tz);
   // `null` tracks "today" live (so a session left open across midnight keeps
   // following the current day); a non-null value pins the viewed day once
diff --git a/src/components/measurement-reminders/__tests__/vorsorge-section.test.tsx b/src/components/measurement-reminders/__tests__/vorsorge-section.test.tsx
index 7bba97edb..da56bb22f 100644
--- a/src/components/measurement-reminders/__tests__/vorsorge-section.test.tsx
+++ b/src/components/measurement-reminders/__tests__/vorsorge-section.test.tsx
@@ -34,7 +34,7 @@ vi.mock("@/hooks/use-measurement-reminders", () => ({
 }));
 
 import { VorsorgeSection, postponeOffsetTargets } from "../vorsorge-section";
-import { DISPLAY_TIMEZONE } from "@/lib/format-locale";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Noon on the day `offsetDays` from today, counted on the DISPLAY zone's
@@ -53,7 +53,7 @@ import { DISPLAY_TIMEZONE } from "@/lib/format-locale";
 function noonOnDisplayDay(offsetDays: number): Date {
   // `en-CA` formats as YYYY-MM-DD, which is the shape we want to add days to.
   const today = new Intl.DateTimeFormat("en-CA", {
-    timeZone: DISPLAY_TIMEZONE,
+    timeZone: DEFAULT_TIMEZONE,
     year: "numeric",
     month: "2-digit",
     day: "2-digit",
@@ -219,7 +219,7 @@ describe("<VorsorgeSection> loading + empty", () => {
     // the wall-clock gap to now exceeds the 24h a rolling delta would key on.
     // Anchor the due instant to noon on the DISPLAY zone's today, not the
     // process clock's. `vitest.config.mts` pins the suite to TZ=UTC while the
-    // component floors in the profile zone (`DISPLAY_TIMEZONE`, Europe/Berlin),
+    // component floors in the profile zone (`DEFAULT_TIMEZONE`, Europe/Berlin),
     // and those two disagree about the date for the two hours before UTC
     // midnight. Anchoring on `setHours` made this case fail every night in
     // that window, which is the same confusion of clocks the assertion exists
diff --git a/src/components/medications/card-parts/displayed-slot-instant.ts b/src/components/medications/card-parts/displayed-slot-instant.ts
index 62eedae0d..a8d633239 100644
--- a/src/components/medications/card-parts/displayed-slot-instant.ts
+++ b/src/components/medications/card-parts/displayed-slot-instant.ts
@@ -1,3 +1,4 @@
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 /**
  * v1.12.3 — resolve the canonical slot instant of the dose a medication
  * card is currently surfacing, so the card's "Genommen" / "Skip" buttons
@@ -138,7 +139,7 @@ export function resolveDisplayedSlotInstant(
     currentWindowStatus,
     nextDueAt,
     now,
-    timeZone = "Europe/Berlin",
+    timeZone = DEFAULT_TIMEZONE,
   } = input;
 
   // The card surfaces the current/overdue dose band — record that band's
diff --git a/src/components/medications/dose-history-ledger.tsx b/src/components/medications/dose-history-ledger.tsx
index e80592a7f..642e4bb7a 100644
--- a/src/components/medications/dose-history-ledger.tsx
+++ b/src/components/medications/dose-history-ledger.tsx
@@ -111,6 +111,7 @@ import {
   type LedgerRow,
 } from "@/components/medications/dose-history-ledger-compute";
 import { apiDelete, apiGet, apiPost, apiPut } from "@/lib/api/api-fetch";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 const DAY_MS = 24 * 60 * 60 * 1000;
 
@@ -182,7 +183,7 @@ export function DoseHistoryLedger({
   // marked — the most-encountered of the set.
   const recordName = useActiveRecordName();
   const queryClient = useQueryClient();
-  const timeZone = user?.timezone || "Europe/Berlin";
+  const timeZone = user?.timezone || DEFAULT_TIMEZONE;
 
   const [addOpen, setAddOpen] = useState(false);
   const [editingEvent, setEditingEvent] = useState<{
diff --git a/src/components/medications/glp1-medication-card.tsx b/src/components/medications/glp1-medication-card.tsx
index b0f22972e..1895daa0a 100644
--- a/src/components/medications/glp1-medication-card.tsx
+++ b/src/components/medications/glp1-medication-card.tsx
@@ -37,6 +37,7 @@ import {
 } from "@/components/medications/detail/supply-runway";
 import { useAuth } from "@/hooks/use-auth";
 import { useMedicationComplianceSummary } from "@/lib/queries/use-medication-compliance-summary";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * v1.4.25 W4d — GLP-1 medication card variant.
@@ -202,7 +203,7 @@ export function Glp1MedicationCard({
   // v1.16.9 — the card reasons in the PROFILE timezone; Berlin stays the
   // last-resort fallback for logged-out mounts and legacy fixtures.
   const { user } = useAuth();
-  const userTz = user?.timezone || "Europe/Berlin";
+  const userTz = user?.timezone || DEFAULT_TIMEZONE;
   const fmt = useFormatters();
   // Issue #490 — the zone `fmt` renders dates in (mirror → Berlin). The
   // weekday name paired with `fmt.dateShort` must be derived in this SAME
diff --git a/src/components/medications/log-intake-dialog.tsx b/src/components/medications/log-intake-dialog.tsx
index 111282526..411ef022f 100644
--- a/src/components/medications/log-intake-dialog.tsx
+++ b/src/components/medications/log-intake-dialog.tsx
@@ -44,6 +44,7 @@ import {
   runLogIntake,
   runUndoIntake,
 } from "@/components/medications/use-medication-intake";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface LogIntakeSchedule extends ScheduleWindowInput {
   label: string | null;
@@ -120,7 +121,7 @@ export function LogIntakeDialog({
   const recordName = useActiveRecordName();
   const queryClient = useQueryClient();
   const { user } = useAuth();
-  const userTz = user?.timezone || "Europe/Berlin";
+  const userTz = user?.timezone || DEFAULT_TIMEZONE;
   const formId = useId();
 
   const [medicationOverride, setMedicationOverride] = useState<string | null>(
diff --git a/src/components/medications/medication-card.tsx b/src/components/medications/medication-card.tsx
index b59b2ac0b..a4d1623c3 100644
--- a/src/components/medications/medication-card.tsx
+++ b/src/components/medications/medication-card.tsx
@@ -34,6 +34,7 @@ import { LogInjectionSiteDialog } from "@/components/medications/log-injection-s
 import { useGlobalExcludedInjectionSites } from "@/lib/medications/use-injection-site-prefs";
 import { useAuth } from "@/hooks/use-auth";
 import type { InjectionSiteKey } from "@/lib/medications/injection-sites";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface Schedule {
   id: string;
@@ -149,7 +150,7 @@ export function MedicationCard({
   // session carries); Berlin stays the last-resort fallback so logged-out
   // mounts and legacy fixtures behave unchanged.
   const { user } = useAuth();
-  const userTz = user?.timezone || "Europe/Berlin";
+  const userTz = user?.timezone || DEFAULT_TIMEZONE;
   const fmt = useFormatters();
   const weekdayLabel = useWeekdayLabel();
   // v1.8.5 — post-dose injection-site prompt state. Holds the intake
diff --git a/src/components/medications/medication-table.tsx b/src/components/medications/medication-table.tsx
index d5089f611..e52c9c92a 100644
--- a/src/components/medications/medication-table.tsx
+++ b/src/components/medications/medication-table.tsx
@@ -43,6 +43,7 @@ import {
 } from "@/lib/queries/use-medication-compliance-summary";
 import { useAuth } from "@/hooks/use-auth";
 import { cn } from "@/lib/utils";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * v1.16.10 — the compact table view for /medications (issue #316 item 3).
@@ -194,7 +195,7 @@ export function MedicationTable({
 }: MedicationTableProps) {
   const { t, locale } = useTranslations();
   const { user } = useAuth();
-  const userTz = user?.timezone || "Europe/Berlin";
+  const userTz = user?.timezone || DEFAULT_TIMEZONE;
   const [sort, setSort] = useState<MedicationTableSort | null>(initialSort);
   const [, forceUpdate] = useReducer((x: number) => x + 1, 0);
 
diff --git a/src/components/medications/take-all-due.ts b/src/components/medications/take-all-due.ts
index aaab08810..d2ff9ac7a 100644
--- a/src/components/medications/take-all-due.ts
+++ b/src/components/medications/take-all-due.ts
@@ -57,6 +57,7 @@ import {
 } from "@/lib/medications/window-status";
 import { resolveDisplayedSlotInstant } from "@/components/medications/card-parts/displayed-slot-instant";
 import type { DoseStatus } from "@/lib/analytics/compliance";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type Translator = (
   key: string,
@@ -142,7 +143,7 @@ export function deriveDueMedications(
   } = {},
 ): DueMedication[] {
   const now = options.now ?? new Date();
-  const tz = options.tz ?? "Europe/Berlin";
+  const tz = options.tz ?? DEFAULT_TIMEZONE;
   const { lateMinutes, missedMinutes } =
     options.thresholds ?? DEFAULT_THRESHOLDS;
 
diff --git a/src/components/settings/account-section/index.tsx b/src/components/settings/account-section/index.tsx
index e07b9c7fa..41eb35e93 100644
--- a/src/components/settings/account-section/index.tsx
+++ b/src/components/settings/account-section/index.tsx
@@ -33,7 +33,7 @@ import { TimeFormatSelect } from "@/components/settings/time-format-select";
 import { DateFormatSelect } from "@/components/settings/date-format-select";
 import { UnitPreferenceSelect } from "@/components/settings/unit-preference-select";
 import { GlucoseUnitSelect } from "@/components/settings/glucose-unit-select";
-import { detectBrowserTimezone } from "@/lib/tz/format";
+import { detectBrowserTimezone, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { apiFetchRaw } from "@/lib/api/api-fetch";
 import {
   describeRejectedProfileField,
@@ -74,7 +74,7 @@ export function AccountSection() {
   const [height, setHeight] = useState<HeightDraft>(EMPTY_HEIGHT_DRAFT);
   const [dateOfBirth, setDateOfBirth] = useState("");
   const [gender, setGender] = useState<string>("");
-  const [timezone, setTimezone] = useState<string>("Europe/Berlin");
+  const [timezone, setTimezone] = useState<string>(DEFAULT_TIMEZONE);
   // v1.7.0 — optional patient-identity fields for the health-record export.
   const [fullName, setFullName] = useState("");
   const [insurerName, setInsurerName] = useState("");
@@ -84,7 +84,7 @@ export function AccountSection() {
     height: EMPTY_HEIGHT_DRAFT,
     dateOfBirth: "",
     gender: "",
-    timezone: "Europe/Berlin",
+    timezone: DEFAULT_TIMEZONE,
     fullName: "",
     insurerName: "",
     insuranceNumber: "",
diff --git a/src/components/settings/managed-record-settings-section.tsx b/src/components/settings/managed-record-settings-section.tsx
index 72639967c..fe98c4f5a 100644
--- a/src/components/settings/managed-record-settings-section.tsx
+++ b/src/components/settings/managed-record-settings-section.tsx
@@ -39,6 +39,7 @@ import { Switch } from "@/components/ui/switch";
 import { SettingsCardActions } from "./_card-actions";
 import { SettingsCardHeader } from "./_card-header";
 import { SettingsCard } from "./settings-card";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface ManagedRecordSettingsResponse {
   recordId: string;
@@ -414,7 +415,7 @@ function ProfileSettingsForm({
       <label className="grid gap-1 text-sm" htmlFor="managed-timezone">
         {t("settings.sharedRecord.managedSettings.profile.timezone")}
         <Input
-          defaultValue={asString(settings.timezone, "Europe/Berlin")}
+          defaultValue={asString(settings.timezone, DEFAULT_TIMEZONE)}
           disabled={disabled}
           id="managed-timezone"
           name="timezone"
diff --git a/src/components/settings/timezone-picker.tsx b/src/components/settings/timezone-picker.tsx
index c6d2aed11..3b86a61df 100644
--- a/src/components/settings/timezone-picker.tsx
+++ b/src/components/settings/timezone-picker.tsx
@@ -26,7 +26,7 @@ import { useMemo } from "react";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 import { NativeSelect } from "@/components/ui/native-select";
-import { listSupportedTimezones } from "@/lib/tz/format";
+import { listSupportedTimezones, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { useTranslations } from "@/lib/i18n/context";
 
 // v1.4.27 MB7 / CF-52 — the in-file `NATIVE_SELECT_CLASS` constant
@@ -67,7 +67,7 @@ export function TimezonePicker({
           id={id}
           value={value}
           onChange={(e) => onChange(e.target.value)}
-          placeholder="Europe/Berlin"
+          placeholder={DEFAULT_TIMEZONE}
           autoComplete="off"
         />
         <p className="text-muted-foreground text-xs">{labelHint}</p>
diff --git a/src/lib/ai/coach/cycle-snapshot.ts b/src/lib/ai/coach/cycle-snapshot.ts
index 297496213..1edd20a02 100644
--- a/src/lib/ai/coach/cycle-snapshot.ts
+++ b/src/lib/ai/coach/cycle-snapshot.ts
@@ -41,7 +41,8 @@ import {
 import type { CrossMetricMeasurement } from "@/lib/insights/mood-aggregates";
 import { addDays } from "@/lib/cycle/day-math";
 import { readSourceDayAggregates } from "@/lib/measurements/day-aggregates";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /** Trailing window the phase contrast walks (days). Mirrors the read route. */
 const WINDOW_DAYS = 365;
@@ -200,12 +201,13 @@ export async function buildCycleSnapshotBlock(
       // many readings to hold as objects (#1023). The crosstab only needs
       // each day's sum and count per source and device, which is what the
       // per-source picker decides on, so the fold runs in SQL. Days are the
-      // Berlin days `metricDayMap` keys on.
+      // user's days, the ones `metricDayMap` keys on and the phase days are
+      // dated in.
       readSourceDayAggregates({
         userId,
         types: PHASE_CROSSTAB_METRIC_TYPES,
         since: new Date(Date.parse(`${from}T00:00:00Z`)),
-        timeZone: "Europe/Berlin",
+        timeZone: tz,
       }),
       prisma.user.findUnique({
         where: { id: userId },
@@ -292,6 +294,7 @@ export async function buildCycleSnapshotBlock(
       phaseByDay,
       measurements,
       userPriorityJson: userRow?.sourcePriorityJson ?? null,
+      timeZone: tz,
     }),
   );
 
diff --git a/src/lib/ai/coach/illness-snapshot.ts b/src/lib/ai/coach/illness-snapshot.ts
index d98146aa7..4d9f3659b 100644
--- a/src/lib/ai/coach/illness-snapshot.ts
+++ b/src/lib/ai/coach/illness-snapshot.ts
@@ -17,7 +17,7 @@
 import { prisma } from "@/lib/db";
 import { isIllnessEnabled } from "@/lib/illness/gate";
 import { computeEpisodeCorrelation } from "@/lib/illness/correlation-read";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { decryptFromBytes } from "@/lib/ai/coach/bytes-codec";
 import { sanitizeForPrompt } from "@/lib/insights/sanitize";
 
diff --git a/src/lib/ai/coach/tools/correlations-read.ts b/src/lib/ai/coach/tools/correlations-read.ts
index 9f7d8a8d3..25961bdc8 100644
--- a/src/lib/ai/coach/tools/correlations-read.ts
+++ b/src/lib/ai/coach/tools/correlations-read.ts
@@ -56,6 +56,7 @@ import {
   loadBaselineProfile,
   isDerivedOk,
 } from "@/lib/insights/derived";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 /** Trailing window for the discovery scan — mirrors the insight route. */
@@ -209,7 +210,7 @@ export async function readCoachCorrelations(
       where: { id: userId },
       select: { timezone: true },
     });
-    const tz = userRow?.timezone ?? "Europe/Berlin";
+    const tz = userRow?.timezone ?? DEFAULT_TIMEZONE;
     const since = new Date(Date.now() - WINDOW_DAYS * MS_PER_DAY);
 
     // The channel set is the shared one, so the Coach scans exactly what the
diff --git a/src/lib/ai/coach/workout-evidence-builder.ts b/src/lib/ai/coach/workout-evidence-builder.ts
index 13217bc5e..43e5de48d 100644
--- a/src/lib/ai/coach/workout-evidence-builder.ts
+++ b/src/lib/ai/coach/workout-evidence-builder.ts
@@ -22,6 +22,7 @@ import {
   buildWorkoutEvidence,
   type WorkoutEvidenceInput,
 } from "./workout-evidence";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface WorkoutEvidenceRow {
   sportType: string;
@@ -108,7 +109,7 @@ export function createWorkoutEvidenceBuilder(
         sportType: row.sportType,
         source: row.source,
         startedAt: row.startedAt,
-        timezone: profile?.timezone ?? "Europe/Berlin",
+        timezone: profile?.timezone ?? DEFAULT_TIMEZONE,
         durationSec: row.durationSec,
         totalEnergyKcal: row.totalEnergyKcal,
         totalDistanceM: row.totalDistanceM,
diff --git a/src/lib/ai/prompts/__tests__/briefing-personalisation.test.ts b/src/lib/ai/prompts/__tests__/briefing-personalisation.test.ts
index e214b0613..49734a63b 100644
--- a/src/lib/ai/prompts/__tests__/briefing-personalisation.test.ts
+++ b/src/lib/ai/prompts/__tests__/briefing-personalisation.test.ts
@@ -15,6 +15,7 @@ describe("buildBriefingPersonalisationBlock", () => {
       "user-1",
       null,
       "en",
+      "UTC",
       day("2026-06-27T08:00:00Z"),
     );
     expect(en).toContain("OPENER HINT:");
@@ -22,6 +23,7 @@ describe("buildBriefingPersonalisationBlock", () => {
       "user-1",
       null,
       "de",
+      "UTC",
       day("2026-06-27T08:00:00Z"),
     );
     expect(de).toContain("OPENER-HINWEIS:");
@@ -33,6 +35,7 @@ describe("buildBriefingPersonalisationBlock", () => {
         "user-1",
         null,
         "en",
+        "UTC",
         day(`2026-06-${String(d).padStart(2, "0")}T08:00:00Z`),
       );
       expect(out).not.toContain("NAME:");
@@ -47,6 +50,7 @@ describe("buildBriefingPersonalisationBlock", () => {
         "user-42",
         "Alex Rivera",
         "en",
+        "UTC",
         day(`2026-06-${String(d).padStart(2, "0")}T08:00:00Z`),
       );
       if (out.includes('"Alex"')) withName += 1;
@@ -62,6 +66,7 @@ describe("buildBriefingPersonalisationBlock", () => {
       "user-42",
       "Alex Rivera",
       "en",
+      "UTC",
       day("2026-06-30T08:00:00Z"),
     );
     expect(out).not.toContain("Rivera");
diff --git a/src/lib/ai/prompts/blood-pressure.ts b/src/lib/ai/prompts/blood-pressure.ts
index 0b913a55b..e0cb2db35 100644
--- a/src/lib/ai/prompts/blood-pressure.ts
+++ b/src/lib/ai/prompts/blood-pressure.ts
@@ -68,12 +68,12 @@ export function getBloodPressureUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's blood pressure. Open with what the reading MEANS in plain words — the overall read, not the number (e.g. "calm and right where you want it", "running a touch higher than your usual this week") — then bring in the systolic/diastolic figures right after as support, judged as ONE pair and placed against their own weekly/monthly baseline; never lead with the numbers. Close with one doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the measurement count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zum Blutdruck dieser Person. Beginne mit der BEDEUTUNG in klaren Worten — dem Gesamteindruck, nicht der Zahl (z. B. "ruhig und genau da, wo du ihn haben willst", "diese Woche einen Tick höher als sonst") — und bring danach die systolischen/diastolischen Werte als Beleg, als EIN Paar beurteilt und gegen die eigene Wochen-/Monats-Baseline eingeordnet; führe nie mit den Werten. Schließe nur dann mit einem machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Messanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/bmi.ts b/src/lib/ai/prompts/bmi.ts
index 9816b1412..5d6727a10 100644
--- a/src/lib/ai/prompts/bmi.ts
+++ b/src/lib/ai/prompts/bmi.ts
@@ -55,12 +55,12 @@ export function getBmiUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's BMI. Open with what its band placement MEANS in plain words — where it sits and whether that's holding or shifting, not the number (e.g. "sitting comfortably in the healthy band", "edging toward the next band up") — then bring in the current value and WHO band right after as support, saying against their own weekly/monthly baseline whether it has crossed a band or is nearing a boundary; never lead with the value. Close with one doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Leave the weight pace to the weight card. Judge confidence from the measurement count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zum BMI dieser Person. Beginne mit der BEDEUTUNG der Bandlage in klaren Worten — wo er liegt und ob das hält oder sich verschiebt, nicht der Zahl (z. B. "liegt bequem im gesunden Band", "nähert sich dem nächsten Band") — und bring danach den aktuellen Wert und das WHO-Band als Beleg, samt Angabe gegen die eigene Wochen-/Monats-Baseline, ob er ein Band gewechselt hat oder sich einer Bandgrenze nähert; führe nie mit dem Wert. Schließe nur dann mit einem machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Das Gewichtstempo bleibt der Gewichts-Karte überlassen. Konfidenz aus Messanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/general-status.ts b/src/lib/ai/prompts/general-status.ts
index 33b4755b4..2f3e27dde 100644
--- a/src/lib/ai/prompts/general-status.ts
+++ b/src/lib/ai/prompts/general-status.ts
@@ -63,12 +63,12 @@ export function getGeneralStatusUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short overall assessment across the available health metrics. Open with the overall read in plain words — how things are looking taken together, not a number (e.g. "a steady stretch across the board", "one thing standing out this week") — then bring in the one or two metrics that stand out as support, each placed against the person's own weekly/monthly baseline; never lead with a value. Close with the single most important doable step only when the overall picture genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the measurement count, density and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Gesamteinschätzung über die verfügbaren Gesundheitsdaten. Beginne mit dem Gesamteindruck in klaren Worten — wie es zusammengenommen aussieht, nicht mit einer Zahl (z. B. "über alles hinweg eine ruhige Phase", "diese Woche sticht eine Sache heraus") — und bring danach die ein bis zwei auffälligsten Metriken als Beleg, jeweils gegen die eigene Wochen-/Monats-Baseline eingeordnet; führe nie mit einem Wert. Schließe nur dann mit dem einen wichtigsten machbaren Schritt, wenn das Gesamtbild wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Messanzahl, Dichte und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/insight-generator.ts b/src/lib/ai/prompts/insight-generator.ts
index 71bfc8ee8..3fd4c4c37 100644
--- a/src/lib/ai/prompts/insight-generator.ts
+++ b/src/lib/ai/prompts/insight-generator.ts
@@ -47,6 +47,7 @@ import {
   outlookContract,
   formattingContract,
 } from "./shared-contracts";
+import { userDayKey } from "@/lib/tz/format";
 
 /**
  * Stable identifier for the active system prompt revision.
@@ -63,16 +64,6 @@ import {
  */
 export const PROMPT_VERSION = "5.1.0" as const;
 
-/** Europe/Berlin YYYY-MM-DD day key — the rotation boundary for the opener. */
-function berlinDayKey(now: Date): string {
-  return new Intl.DateTimeFormat("en-CA", {
-    timeZone: "Europe/Berlin",
-    year: "numeric",
-    month: "2-digit",
-    day: "2-digit",
-  }).format(now);
-}
-
 /**
  * v1.22 (W6) — the daily-briefing personalization SYSTEM CONTEXT block.
  *
@@ -92,9 +83,11 @@ export function buildBriefingPersonalisationBlock(
   userId: string,
   displayName: string | null,
   locale: Locale,
+  /** The user's zone: the opener rotates at the user's own midnight. */
+  timeZone: string,
   now: Date = new Date(),
 ): string {
-  const dayKey = berlinDayKey(now);
+  const dayKey = userDayKey(now, timeZone);
   const hint = openerArchetypeHint(`${userId}:briefing:${dayKey}`, locale);
   const firstName = firstNameFromDisplayName(displayName);
   const useName =
diff --git a/src/lib/ai/prompts/medication-compliance.ts b/src/lib/ai/prompts/medication-compliance.ts
index 8db71fd31..22d54bdd4 100644
--- a/src/lib/ai/prompts/medication-compliance.ts
+++ b/src/lib/ai/prompts/medication-compliance.ts
@@ -57,12 +57,12 @@ export function getMedicationComplianceUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's medication adherence. Open with what it MEANS in plain words — how reliable the routine is looking, not the number (e.g. "your routine's been rock-solid", "a few doses have slipped lately") — then bring in the recent rate right after as support, placed against their own baseline (compliance7 vs compliance30); never lead with the value. Close with one doable, blame-free step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the event count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zur Einnahmetreue dieser Person. Beginne mit der BEDEUTUNG in klaren Worten — wie verlässlich die Routine aussieht, nicht der Zahl (z. B. "deine Routine sitzt richtig gut", "zuletzt sind ein paar Dosen durchgerutscht") — und bring danach die jüngste Rate als Beleg, gegen die eigene Baseline eingeordnet (compliance7 vs. compliance30); führe nie mit dem Wert. Schließe nur dann mit einem machbaren, wertfreien Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Ereignisanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/metric-archetypes.ts b/src/lib/ai/prompts/metric-archetypes.ts
index 1d8f45279..228e1bbca 100644
--- a/src/lib/ai/prompts/metric-archetypes.ts
+++ b/src/lib/ai/prompts/metric-archetypes.ts
@@ -165,12 +165,12 @@ export function getMetricArchetypeUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's ${meta.displayName.toLowerCase()}. Open with what it MEANS in plain words — the read, not the number (e.g. "running a little calmer than usual", "steady and right where it's been") — then bring in ONE concrete number from the snapshot right after as support, placed against their own weekly/monthly baseline; never lead with the value. Close with one doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the measurement count and recency.${ctxBlock}${extraBlock}${interpBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zu ${meta.displayName} dieser Person. Beginne mit der BEDEUTUNG in klaren Worten — dem Eindruck, nicht der Zahl (z. B. "läuft gerade etwas ruhiger als sonst", "stabil und genau da, wo es war") — und bring danach EINE konkrete Zahl aus dem Snapshot als Beleg, gegen die eigene Wochen-/Monats-Baseline eingeordnet; führe nie mit dem Wert. Schließe nur dann mit einem machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Messanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}${interpBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/mood.ts b/src/lib/ai/prompts/mood.ts
index 7e10f6f37..2af12d2a6 100644
--- a/src/lib/ai/prompts/mood.ts
+++ b/src/lib/ai/prompts/mood.ts
@@ -57,12 +57,12 @@ export function getMoodUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's mood. Open with what the recent pattern MEANS in plain words — the read, not the number (e.g. "a brighter stretch than usual", "steady, holding where it's been") — then bring in the recent level right after as support, placed against their own weekly/monthly baseline; never lead with the value. Close with one kind, doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the entry count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zur Stimmung dieser Person. Beginne mit der BEDEUTUNG des jüngsten Musters in klaren Worten — dem Eindruck, nicht der Zahl (z. B. "eine hellere Phase als sonst", "stabil, hält sich, wo sie war") — und bring danach das jüngste Niveau als Beleg, gegen die eigene Wochen-/Monats-Baseline eingeordnet; führe nie mit dem Wert. Schließe nur dann mit einem freundlichen, machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Eintragsanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/pulse.ts b/src/lib/ai/prompts/pulse.ts
index 5249b3164..eca3b1354 100644
--- a/src/lib/ai/prompts/pulse.ts
+++ b/src/lib/ai/prompts/pulse.ts
@@ -55,12 +55,12 @@ export function getPulseUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's resting pulse. Open with what it MEANS in plain words — the read, not the number (e.g. "running a little calmer than usual", "steady, right where it's been") — then bring in ONE concrete number from the snapshot right after as support, placed against their own weekly/monthly baseline; never lead with the value. Close with one doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the measurement count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zum Ruhepuls dieser Person. Beginne mit der BEDEUTUNG in klaren Worten — dem Eindruck, nicht der Zahl (z. B. "läuft gerade etwas ruhiger als sonst", "stabil, genau da, wo er war") — und bring danach EINE konkrete Zahl aus dem Snapshot als Beleg, gegen die eigene Wochen-/Monats-Baseline eingeordnet; führe nie mit dem Wert. Schließe nur dann mit einem machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Messanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/ai/prompts/weight.ts b/src/lib/ai/prompts/weight.ts
index b0f201352..8ef53abc4 100644
--- a/src/lib/ai/prompts/weight.ts
+++ b/src/lib/ai/prompts/weight.ts
@@ -63,12 +63,12 @@ export function getWeightUserPrompt(
       ? `\nOPENER HINT: ${openerHint}`
       : "";
   if (instructionLocale(locale) === "en") {
-    return `Date: ${todayKey} (Europe/Berlin)${openerLine}
+    return `Date: ${todayKey}${openerLine}
 Write one short assessment of this person's weight. Open with what the trend MEANS in plain words — the direction and momentum, not the number (e.g. "easing down steadily", "holding right where it's settled") — then bring in ONE concrete number from the snapshot right after as support, read as a continuous trend and pace against their own weekly/monthly baseline (kg/week, plateau, milestone — not the single value, and not the WHO band, which the BMI card covers); never lead with the value. Close with one doable step only when the finding genuinely implies one; when nothing is, skip the step rather than manufacture filler. Judge confidence from the measurement count and recency.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
   }
-  return `Datum: ${todayKey} (Europe/Berlin)${openerLine}
+  return `Datum: ${todayKey}${openerLine}
 Schreibe eine kurze Einschätzung zum Gewicht dieser Person. Beginne mit der BEDEUTUNG in klaren Worten — Richtung und Tempo, nicht der Zahl (z. B. "geht ruhig nach unten", "hält sich genau da, wo es sich eingependelt hat") — und bring danach EINE konkrete Zahl aus dem Snapshot als Beleg, als kontinuierlichen Trend und Tempo gegen die eigene Wochen-/Monats-Baseline gelesen (kg/Woche, Plateau, Meilenstein — nicht der Einzelwert und nicht das WHO-Band, das die BMI-Karte trägt); führe nie mit dem Wert. Schließe nur dann mit einem machbaren Schritt, wenn der Befund wirklich einen hergibt; ist nichts umsetzbar, lass den Schritt weg statt Fülltext zu erfinden. Konfidenz aus Messanzahl und Aktualität ableiten.${ctxBlock}${extraBlock}
 
 ${snapshotJson}`;
diff --git a/src/lib/analytics/__tests__/intraday-pulse-io.test.ts b/src/lib/analytics/__tests__/intraday-pulse-io.test.ts
index b1994537f..b62f2dd68 100644
--- a/src/lib/analytics/__tests__/intraday-pulse-io.test.ts
+++ b/src/lib/analytics/__tests__/intraday-pulse-io.test.ts
@@ -29,7 +29,6 @@ import {
   resolveRestingPulseSeries,
   type PulseSample,
 } from "@/lib/analytics/resting-pulse";
-import { toBerlinDayKey } from "@/lib/tz/resolver";
 import { userDayKey } from "@/lib/tz/format";
 import { percentile } from "@/lib/insights/strain-score";
 import { localDayWindow } from "@/lib/tz/local-day";
@@ -96,7 +95,7 @@ describe("loadIntradayPulse", () => {
     // Sanity: under the OLD hardcoded-Berlin bug all six samples fall on the
     // SAME Berlin day — proving the two tz groupings genuinely diverge.
     const berlinDays = new Set(
-      [...dayA, ...dayB].map((d) => toBerlinDayKey(d)),
+      [...dayA, ...dayB].map((d) => userDayKey(d, "Europe/Berlin")),
     );
     expect(berlinDays.size).toBe(1);
 
@@ -151,7 +150,7 @@ describe("loadIntradayPulse", () => {
     const buggyBerlin = resolveRestingPulseSeries({
       restingSamples: [],
       pulseSamples: pulseHistory as PulseSample[],
-      dayKeyOf: (d) => toBerlinDayKey(d),
+      dayKeyOf: (d) => userDayKey(d, "Europe/Berlin"),
     });
     const buggyBaseline = median(
       buggyBerlin.series.slice(-30).map((p) => p.value),
diff --git a/src/lib/analytics/__tests__/resting-pulse.test.ts b/src/lib/analytics/__tests__/resting-pulse.test.ts
index af7e7fc6b..71598112d 100644
--- a/src/lib/analytics/__tests__/resting-pulse.test.ts
+++ b/src/lib/analytics/__tests__/resting-pulse.test.ts
@@ -13,6 +13,9 @@ import {
   type PulseSample,
 } from "../resting-pulse";
 import { zonedWallClockToUtc } from "@/lib/tz/wall-clock";
+import { userDayKey } from "@/lib/tz/format";
+
+const DAY_KEY = (d: Date) => userDayKey(d, "Europe/Berlin");
 
 /**
  * The zone `deriveRestingProxyFromPulse` buckets in by default
@@ -69,7 +72,7 @@ describe("deriveRestingProxyFromPulse", () => {
         ),
       );
     }
-    const proxy = deriveRestingProxyFromPulse(samples);
+    const proxy = deriveRestingProxyFromPulse(samples, DAY_KEY);
     expect(proxy).toHaveLength(1);
     // 510 samples, only 10 below ~80 — but the 20th percentile of the
     // whole day still lands in the workout band because 98 % of samples
@@ -94,7 +97,7 @@ describe("deriveRestingProxyFromPulse", () => {
     for (let i = 0; i < 10; i++) {
       samples.push(day(`2026-06-02T18:${String(i).padStart(2, "0")}:00`, 150));
     }
-    const proxy = deriveRestingProxyFromPulse(samples);
+    const proxy = deriveRestingProxyFromPulse(samples, DAY_KEY);
     expect(proxy).toHaveLength(1);
     // 20th percentile sits in the low band, well below the workout reads.
     expect(proxy[0].value).toBeLessThanOrEqual(80);
@@ -112,7 +115,7 @@ describe("deriveRestingProxyFromPulse", () => {
       day("2026-06-01T12:00:00", 64),
       day("2026-06-01T20:00:00", 68),
     ];
-    const proxy = deriveRestingProxyFromPulse(samples);
+    const proxy = deriveRestingProxyFromPulse(samples, DAY_KEY);
     expect(proxy).toHaveLength(2);
     expect(proxy[0].measuredAt.getTime()).toBeLessThan(
       proxy[1].measuredAt.getTime(),
@@ -125,7 +128,9 @@ describe("deriveRestingProxyFromPulse", () => {
     const oneSampleWorkoutDay: PulseSample[] = [
       day("2026-06-05T18:00:00", 165),
     ];
-    expect(deriveRestingProxyFromPulse(oneSampleWorkoutDay)).toEqual([]);
+    expect(deriveRestingProxyFromPulse(oneSampleWorkoutDay, DAY_KEY)).toEqual(
+      [],
+    );
 
     // A day below the min-sample floor is dropped; only the day with enough
     // samples contributes.
@@ -135,13 +140,13 @@ describe("deriveRestingProxyFromPulse", () => {
       day("2026-06-06T08:00:00", 62),
       day("2026-06-06T09:00:00", 64),
     ];
-    const proxy = deriveRestingProxyFromPulse(mixed);
+    const proxy = deriveRestingProxyFromPulse(mixed, DAY_KEY);
     expect(proxy).toHaveLength(1);
     expect(proxy[0].value).toBeLessThan(150);
   });
 
   it("returns empty for no samples", () => {
-    expect(deriveRestingProxyFromPulse([])).toEqual([]);
+    expect(deriveRestingProxyFromPulse([], DAY_KEY)).toEqual([]);
   });
 });
 
@@ -158,6 +163,7 @@ describe("resolveRestingPulseSeries", () => {
       day("2026-06-01T18:00:00", 150),
     );
     const { series, which } = resolveRestingPulseSeries({
+      dayKeyOf: DAY_KEY,
       restingSamples,
       pulseSamples,
     });
@@ -174,6 +180,7 @@ describe("resolveRestingPulseSeries", () => {
       day("2026-06-01T18:00:00", 150),
     ];
     const { series, which } = resolveRestingPulseSeries({
+      dayKeyOf: DAY_KEY,
       restingSamples: [],
       pulseSamples,
     });
@@ -184,7 +191,11 @@ describe("resolveRestingPulseSeries", () => {
 
   it("reports 'none' when neither series has data", () => {
     expect(
-      resolveRestingPulseSeries({ restingSamples: [], pulseSamples: [] }),
+      resolveRestingPulseSeries({
+        dayKeyOf: DAY_KEY,
+        restingSamples: [],
+        pulseSamples: [],
+      }),
     ).toEqual({ series: [], which: "none" });
   });
 
@@ -218,6 +229,7 @@ describe("resolveRestingPulseSeries", () => {
     }
 
     const { series, which } = resolveRestingPulseSeries({
+      dayKeyOf: DAY_KEY,
       restingSamples,
       pulseSamples,
     });
@@ -249,6 +261,7 @@ describe("resolveRestingPulseSeries", () => {
     // A native day must not be second-guessed by the proxy, and a native
     // account with no gap days keeps the honest 'resting' label.
     const { series, which } = resolveRestingPulseSeries({
+      dayKeyOf: DAY_KEY,
       restingSamples: [day("2026-06-01T04:00:00", 58)],
       pulseSamples: [
         day("2026-06-01T07:00:00", 80),
@@ -276,7 +289,7 @@ describe("deriveRestingProxyFromPulse — the day bucket is the PROFILE day", ()
       day("2026-06-02T15:00:00", 68),
       day("2026-06-02T21:00:00", 70),
     ];
-    const proxy = deriveRestingProxyFromPulse(samples);
+    const proxy = deriveRestingProxyFromPulse(samples, DAY_KEY);
     expect(proxy).toHaveLength(1);
   });
 
@@ -293,7 +306,7 @@ describe("deriveRestingProxyFromPulse — the day bucket is the PROFILE day", ()
       day("2026-06-02T00:30:00", 82),
       day("2026-06-02T00:50:00", 84),
     ];
-    const proxy = deriveRestingProxyFromPulse(samples);
+    const proxy = deriveRestingProxyFromPulse(samples, DAY_KEY);
     expect(proxy).toHaveLength(2);
   });
 
diff --git a/src/lib/analytics/mood-prognosis/load.ts b/src/lib/analytics/mood-prognosis/load.ts
index 930326756..58376eb85 100644
--- a/src/lib/analytics/mood-prognosis/load.ts
+++ b/src/lib/analytics/mood-prognosis/load.ts
@@ -22,7 +22,7 @@
  */
 import { prisma } from "@/lib/db";
 import { decodeKeyList } from "@/lib/mood/context-vocabulary";
-import { DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { resolveLinkedDayFigures } from "@/lib/mood/linked-context";
 
 import type { PrognosisDayInput } from "./features";
diff --git a/src/lib/analytics/mood-prognosis/read.ts b/src/lib/analytics/mood-prognosis/read.ts
index 480ea9fe5..0c47ddf97 100644
--- a/src/lib/analytics/mood-prognosis/read.ts
+++ b/src/lib/analytics/mood-prognosis/read.ts
@@ -22,7 +22,8 @@ import {
   dayKeyAgeInDays,
   isCurrentForTodayClaim,
 } from "@/lib/insights/measurement-freshness";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 import type { FeatureContribution } from "./forecast";
 import { countQualifyingDays } from "./load";
diff --git a/src/lib/analytics/resting-pulse.ts b/src/lib/analytics/resting-pulse.ts
index 652079dae..8f8deaacc 100644
--- a/src/lib/analytics/resting-pulse.ts
+++ b/src/lib/analytics/resting-pulse.ts
@@ -39,7 +39,6 @@
  * Pure & deterministic — unit-tested in `__tests__/resting-pulse.test.ts`.
  */
 import { percentile } from "@/lib/insights/strain-score";
-import { toBerlinDayKey } from "@/lib/tz/resolver";
 
 /** A timestamped heart-rate sample. */
 export interface PulseSample {
@@ -78,12 +77,11 @@ export const RESTING_PROXY_MIN_DAILY_SAMPLES = 3;
 export function deriveRestingProxyFromPulse(
   pulseSamples: ReadonlyArray<PulseSample>,
   /**
-   * Day-key function used to bucket samples. Defaults to Berlin-day so
-   * direct callers / tests stay zero-config; the targets route passes
-   * its own `userDayKey(d, userTz)` so the proxy buckets align with the
-   * route's consistency-strip day buckets for non-Berlin users.
+   * Day-key function used to bucket samples: the user's own day
+   * (`userDayKey(d, userTz)`), so the proxy buckets line up with every other
+   * day bucket the user sees.
    */
-  dayKeyOf: (d: Date) => string = toBerlinDayKey,
+  dayKeyOf: (d: Date) => string,
 ): PulseSample[] {
   if (pulseSamples.length === 0) return [];
   const byDay = new Map<string, { firstAt: Date; values: number[] }>();
@@ -139,10 +137,10 @@ export function deriveRestingProxyFromPulse(
 export function resolveRestingPulseSeries(input: {
   restingSamples: ReadonlyArray<PulseSample>;
   pulseSamples: ReadonlyArray<PulseSample>;
-  /** Optional day-key for the proxy buckets (defaults to Berlin-day). */
-  dayKeyOf?: (d: Date) => string;
+  /** Day key for the proxy buckets: the user's own day. */
+  dayKeyOf: (d: Date) => string;
 }): { series: PulseSample[]; which: "resting" | "proxy" | "none" } {
-  const dayKeyOf = input.dayKeyOf ?? toBerlinDayKey;
+  const dayKeyOf = input.dayKeyOf;
   const byTime = (a: PulseSample, b: PulseSample) =>
     a.measuredAt.getTime() - b.measuredAt.getTime();
 
diff --git a/src/lib/cycle/__tests__/phase-crosstab.test.ts b/src/lib/cycle/__tests__/phase-crosstab.test.ts
index ea8e79147..b9b799b44 100644
--- a/src/lib/cycle/__tests__/phase-crosstab.test.ts
+++ b/src/lib/cycle/__tests__/phase-crosstab.test.ts
@@ -119,7 +119,11 @@ describe("computePhaseMetricCrosstab — MOOD + GLUCOSE outcomes (QA HIGH)", ()
 
   it("surfaces a luteal-vs-follicular GLUCOSE contrast", () => {
     const { phaseByDay, measurements } = contrastFor("BLOOD_GLUCOSE", 105, 92);
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     const glucose = rows.find((r) => r.metricKey === "bloodGlucose");
     expect(glucose).toBeDefined();
     expect(glucose!.display).toBe("glucose");
@@ -133,7 +137,11 @@ describe("computePhaseMetricCrosstab — MOOD + GLUCOSE outcomes (QA HIGH)", ()
       4.4,
       2.6,
     );
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     const mood = rows.find((r) => r.metricKey === "mood");
     expect(mood).toBeDefined();
     expect(mood!.display).toBe("mood");
@@ -164,7 +172,11 @@ describe("computePhaseMetricCrosstab", () => {
       lutealRhr: 63,
       follicularRhr: 57,
     });
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     const rhr = rows.find((r) => r.metricKey === "restingHeartRate");
     expect(rhr).toBeDefined();
     // luteal higher → positive delta (lutealAvg − follicularAvg)
@@ -183,7 +195,11 @@ describe("computePhaseMetricCrosstab", () => {
       lutealRhr: 63,
       follicularRhr: 57,
     });
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     expect(rows).toEqual([]);
   });
 
@@ -193,7 +209,11 @@ describe("computePhaseMetricCrosstab", () => {
       lutealRhr: 60,
       follicularRhr: 60,
     });
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     // No real separation → nothing clears p < 0.05.
     expect(
       rows.find((r) => r.metricKey === "restingHeartRate"),
@@ -216,7 +236,11 @@ describe("computePhaseMetricCrosstab", () => {
       phaseByDay.set(key, i % 2 === 0 ? "MENSTRUAL" : "OVULATORY");
       measurements.push(m("RESTING_HEART_RATE", key, 200));
     }
-    const rows = computePhaseMetricCrosstab({ phaseByDay, measurements });
+    const rows = computePhaseMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      phaseByDay,
+      measurements,
+    });
     const rhr = rows.find((r) => r.metricKey === "restingHeartRate")!;
     // Means reflect only the luteal/follicular days, not the 200-bpm noise.
     expect(rhr.lutealAvg).toBeLessThan(70);
@@ -226,6 +250,7 @@ describe("computePhaseMetricCrosstab", () => {
   it("returns empty for an empty phase map", () => {
     expect(
       computePhaseMetricCrosstab({
+        timeZone: "Europe/Berlin",
         phaseByDay: new Map(),
         measurements: [m("RESTING_HEART_RATE", "2026-03-01", 60)],
       }),
@@ -287,6 +312,7 @@ describe("discoverPhaseCorrelations", () => {
       );
     }
     const result = discoverPhaseCorrelations({
+      timeZone: "Europe/Berlin",
       phaseByDay,
       measurements,
       locale: "en",
@@ -320,6 +346,7 @@ describe("discoverPhaseCorrelations", () => {
       measurements.push(m("RESTING_HEART_RATE", key, 60)); // constant → no variance
     }
     const result = discoverPhaseCorrelations({
+      timeZone: "Europe/Berlin",
       phaseByDay,
       measurements,
       locale: "en",
diff --git a/src/lib/cycle/phase-crosstab.ts b/src/lib/cycle/phase-crosstab.ts
index 3971a22ca..950beb724 100644
--- a/src/lib/cycle/phase-crosstab.ts
+++ b/src/lib/cycle/phase-crosstab.ts
@@ -183,6 +183,8 @@ export function computePhaseMetricCrosstab(args: {
   measurements: CrossMetricMeasurement[];
   /** The user's source-priority blob (threaded into `metricDayMap`). */
   userPriorityJson?: unknown;
+  /** The user's zone, the calendar the phase days are keyed on. */
+  timeZone: string;
 }): PhaseMetricCrosstabRow[] {
   const { phaseByDay, measurements } = args;
   const userPriorityJson = args.userPriorityJson ?? null;
@@ -198,7 +200,12 @@ export function computePhaseMetricCrosstab(args: {
       (typeof PHASE_CROSSTAB_METRICS)[PhaseCrosstabMetricKey],
     ]
   >) {
-    const metricByDay = metricDayMap(measurements, cfg.type, userPriorityJson);
+    const metricByDay = metricDayMap(
+      measurements,
+      cfg.type,
+      userPriorityJson,
+      args.timeZone,
+    );
     if (metricByDay.size === 0) continue;
 
     const lutealVals: number[] = [];
@@ -293,6 +300,8 @@ export function discoverPhaseCorrelations(args: {
   phaseByDay: Map<string, CyclePhase>;
   measurements: CrossMetricMeasurement[];
   userPriorityJson?: unknown;
+  /** The user's zone, the calendar the phase days are keyed on. */
+  timeZone: string;
   /**
    * The reader's locale. Each surviving row carries a finished `interpretation`
    * sentence that the cycle route serves straight to the client, so it has to
@@ -307,7 +316,12 @@ export function discoverPhaseCorrelations(args: {
   const phaseSeries = buildPhaseDiscoverySeries(phaseByDay);
   const outcomeSeries: NamedSeries[] = [];
   for (const [, cfg] of Object.entries(PHASE_CROSSTAB_METRICS)) {
-    const byDay = metricDayMap(measurements, cfg.type, userPriorityJson);
+    const byDay = metricDayMap(
+      measurements,
+      cfg.type,
+      userPriorityJson,
+      args.timeZone,
+    );
     if (byDay.size === 0) continue;
     const points: DailySeriesPoint[] = [];
     for (const [day, value] of byDay) points.push({ day, value });
diff --git a/src/lib/cycle/prediction-refresh.ts b/src/lib/cycle/prediction-refresh.ts
index c324788e5..02641f861 100644
--- a/src/lib/cycle/prediction-refresh.ts
+++ b/src/lib/cycle/prediction-refresh.ts
@@ -25,7 +25,8 @@ import {
 import { persistPredictionCache } from "@/lib/cycle/prediction-cache";
 import { addDays } from "@/lib/cycle/day-math";
 import { BBT_WINDOW } from "@/lib/cycle/types";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /** Day-log lookback: the calendar's default past span or the symptothermal
  *  window, whichever reaches further back. */
diff --git a/src/lib/daily/daily-briefing-push.ts b/src/lib/daily/daily-briefing-push.ts
index ef62c1f24..a64266730 100644
--- a/src/lib/daily/daily-briefing-push.ts
+++ b/src/lib/daily/daily-briefing-push.ts
@@ -33,7 +33,7 @@ import { resolveJobLocale } from "@/lib/i18n/job-locale";
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 import type { ServerTranslator } from "@/lib/i18n/server-translator";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
-import { userDayKey } from "@/lib/tz/format";
+import { userDayKey, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { dispatchNotification } from "@/lib/notifications/dispatcher";
 import { loadDailyDigest } from "@/lib/daily/load-digest";
 import type { DailyDigest } from "@/lib/daily/digest";
@@ -175,7 +175,7 @@ export async function maybeDispatchDailyBriefing(
       return "opted-out";
     }
 
-    const tz = user.timezone || "Europe/Berlin";
+    const tz = user.timezone || DEFAULT_TIMEZONE;
     const { hour } = wallClockInTz(now, tz);
     if (
       hour < DAILY_BRIEFING_MORNING_EARLIEST_HOUR ||
diff --git a/src/lib/doctor-report-pdf-core.ts b/src/lib/doctor-report-pdf-core.ts
index fc7aaaa4b..bdb689fea 100644
--- a/src/lib/doctor-report-pdf-core.ts
+++ b/src/lib/doctor-report-pdf-core.ts
@@ -22,7 +22,6 @@ import {
 import { getUnitForType } from "./validations/measurement";
 import {
   makeFormatters,
-  DISPLAY_TIMEZONE,
   type DateFormatPreference,
   type TimeFormatPreference,
 } from "./format-locale";
@@ -39,6 +38,7 @@ import {
   pdfCursorState,
   type DoctorReportPdfRenderContext,
 } from "./doctor-report-pdf/render-context";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 type T = (key: string, params?: Record<string, string | number>) => string;
 
@@ -236,7 +236,7 @@ export function buildDoctorReportPdfDocument(
     formatters.number(value, decimals);
   const fmtDate = (iso: string) => formatters.date(iso);
   const footerTz =
-    userTz && isValidTimezone(userTz) ? userTz : DISPLAY_TIMEZONE;
+    userTz && isValidTimezone(userTz) ? userTz : DEFAULT_TIMEZONE;
 
   const unitFor = (type: string): string => {
     const staticUnit = DOCTOR_REPORT_TYPE_UNIT_KEYS[type];
diff --git a/src/lib/doctor-report/collect.ts b/src/lib/doctor-report/collect.ts
index 502480dba..0effcf3ba 100644
--- a/src/lib/doctor-report/collect.ts
+++ b/src/lib/doctor-report/collect.ts
@@ -61,7 +61,7 @@ import { loadDenseMeasurementBuckets } from "./dense-buckets";
 import { summariseDenseBuckets } from "./measurement-series";
 import { emptyGlucoseClinical } from "./glucose-panel";
 import { buildAdministrationLedger, buildGlp1Block } from "./medications";
-import { validTimezoneOr } from "@/lib/tz/format";
+import { validTimezoneOr, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import {
   loadAllergies,
   loadAnamnesis,
@@ -147,7 +147,7 @@ export async function collectDoctorReportData(
       insurerIkNumber: true,
     },
   });
-  const reportTz = validTimezoneOr(userProfile?.timezone, "Europe/Berlin");
+  const reportTz = validTimezoneOr(userProfile?.timezone, DEFAULT_TIMEZONE);
 
   const aggregateDenseTypes = days > DENSE_REPORT_RAW_WINDOW_DAYS;
   const densePulse = aggregateDenseTypes && !excluded.includes("PULSE");
diff --git a/src/lib/encounters/service.ts b/src/lib/encounters/service.ts
index 88d3b311f..cb324a443 100644
--- a/src/lib/encounters/service.ts
+++ b/src/lib/encounters/service.ts
@@ -27,8 +27,7 @@ import {
   reanchorAppointmentReminder,
   type AppointmentReminderInput,
 } from "@/lib/encounters/appointment-reminder";
-
-const DEFAULT_TIMEZONE = "Europe/Berlin";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * The record owner's zone AND language, read together.
diff --git a/src/lib/format-locale.ts b/src/lib/format-locale.ts
index bbe587dfe..73a2bc669 100644
--- a/src/lib/format-locale.ts
+++ b/src/lib/format-locale.ts
@@ -9,22 +9,14 @@
  * (jobs, API routes, PDF generation) pass the locale explicitly through
  * `makeFormatters(locale)`. The optional second argument carries the
  * per-user timezone (v1.4.25 W7) — when omitted the formatter falls back
- * to `DISPLAY_TIMEZONE` so legacy callers keep rendering in Europe/Berlin.
+ * to `DEFAULT_TIMEZONE` so legacy callers keep rendering in the default zone.
  *
  * UTC is always preserved in the database.
  */
 
 import type { Locale } from "./i18n/config";
 import { getDateTimeFormat, getNumberFormat } from "./intl/formatter-cache";
-import { isValidTimezone } from "./tz/format";
-
-/**
- * Fallback timezone for surfaces that are not yet user-scoped (admin
- * tables, audit log viewer, anything without a resolved user). User-
- * scoped surfaces should call `makeFormatters(locale, userTz)` with
- * the value from `resolveUserTimezone()`.
- */
-export const DISPLAY_TIMEZONE = "Europe/Berlin";
+import { DEFAULT_TIMEZONE, isValidTimezone } from "./tz/format";
 
 /**
  * Map our short locale ("de" / "en") to a full BCP-47 tag suitable for
@@ -185,7 +177,7 @@ export interface Formatters {
  * `useFormatters()` silently took the default. A caller with nothing to
  * pass writes "AUTO" and thereby says so. Same for `userTz`: pass the
  * resolved profile zone, or an explicit `undefined` to accept the
- * `DISPLAY_TIMEZONE` fallback.
+ * `DEFAULT_TIMEZONE` fallback.
  */
 export function makeFormatters(
   locale: Locale,
@@ -199,7 +191,7 @@ export function makeFormatters(
   // every page that renders a timestamp. Validate here (cheap — the
   // probe memoises successful constructions) and fall back to Berlin,
   // matching `formatInUserTz` / `hourInTz` / `isNearUtc`.
-  const tz = userTz && isValidTimezone(userTz) ? userTz : DISPLAY_TIMEZONE;
+  const tz = userTz && isValidTimezone(userTz) ? userTz : DEFAULT_TIMEZONE;
   const hourOpts = hourCycleOptions(timeFormat);
   // Field-order locale for the numeric date renderers. AUTO keeps the
   // user's own locale; DMY/MDY/YMD pin a canonical locale whose default
diff --git a/src/lib/gamification/__tests__/achievements.test.ts b/src/lib/gamification/__tests__/achievements.test.ts
index 67776d0aa..215f22bac 100644
--- a/src/lib/gamification/__tests__/achievements.test.ts
+++ b/src/lib/gamification/__tests__/achievements.test.ts
@@ -6,7 +6,6 @@ import {
   bridgeFrozenStreakGaps,
   calculateLongestStreak,
   evaluateAchievementsWithCompletionDates,
-  getUniqueBerlinDays,
   type AchievementMetrics,
   type AchievementProgress,
   type EarnabilityFlags,
@@ -68,16 +67,6 @@ const NONE_EARNABLE: EarnabilityFlags = {
 };
 
 describe("gamification achievements", () => {
-  it("deduplicates and sorts day keys", () => {
-    const days = getUniqueBerlinDays([
-      new Date("2026-02-11T12:00:00Z"),
-      new Date("2026-02-10T12:00:00Z"),
-      new Date("2026-02-11T16:00:00Z"),
-    ]);
-
-    expect(days).toEqual(["2026-02-10", "2026-02-11"]);
-  });
-
   it("calculates the longest streak", () => {
     expect(
       calculateLongestStreak([
diff --git a/src/lib/gamification/__tests__/expansion-metrics.test.ts b/src/lib/gamification/__tests__/expansion-metrics.test.ts
index 6dde24ab6..435b66940 100644
--- a/src/lib/gamification/__tests__/expansion-metrics.test.ts
+++ b/src/lib/gamification/__tests__/expansion-metrics.test.ts
@@ -7,7 +7,7 @@ import {
   getHiddenMetrics,
   getMoodMetrics,
 } from "@/lib/gamification/expansion-metrics";
-import { toBerlinDayKey } from "@/lib/gamification/achievements";
+import { userDayKey } from "@/lib/tz/format";
 
 // Mirror of the module-private Berlin-hour derivation so the parity test can
 // build a CORRECT precomputed `measurementHours` array.
@@ -408,7 +408,7 @@ describe("achievements Intl precompute — byte-identical parity", () => {
   ];
 
   const measurementDayKeys = measurements.map((m) =>
-    toBerlinDayKey(m.measuredAt),
+    userDayKey(m.measuredAt, "Europe/Berlin"),
   );
   const measurementHours = measurements.map((m) => berlinHour(m.measuredAt));
 
diff --git a/src/lib/gamification/achievements.ts b/src/lib/gamification/achievements.ts
index 81c90fb41..1e2862333 100644
--- a/src/lib/gamification/achievements.ts
+++ b/src/lib/gamification/achievements.ts
@@ -1,5 +1,3 @@
-const BERLIN_TIMEZONE = "Europe/Berlin";
-
 export const GAMIFICATION_ROLLOUT_AT = new Date("2026-02-20T00:00:00.000Z");
 
 export type AchievementMetricKey =
@@ -233,13 +231,6 @@ export interface AchievementSummary {
   nextAchievement: AchievementProgress | null;
 }
 
-const DAY_KEY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: BERLIN_TIMEZONE,
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
-
 const STREAK_TARGETS = [1, 7, 30, 180, 360] as const;
 
 function buildStreakAchievements(config: {
@@ -723,23 +714,6 @@ function dayKeyToNumber(dayKey: string): number {
   return Date.UTC(year, month - 1, day) / 86_400_000;
 }
 
-export function toBerlinDayKey(date: Date): string {
-  const parts = DAY_KEY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not format date");
-  }
-
-  return `${year}-${month}-${day}`;
-}
-
-export function getUniqueBerlinDays(dates: Date[]): string[] {
-  return Array.from(new Set(dates.map((date) => toBerlinDayKey(date)))).sort();
-}
-
 export function calculateLongestStreak(dayKeys: string[]): number {
   if (dayKeys.length === 0) return 0;
 
diff --git a/src/lib/i18n/context.tsx b/src/lib/i18n/context.tsx
index 446f60042..87f7f80b2 100644
--- a/src/lib/i18n/context.tsx
+++ b/src/lib/i18n/context.tsx
@@ -14,7 +14,6 @@ import {
 import { apiFetchRaw } from "@/lib/api/api-fetch";
 import { locales, defaultLocale, type Locale } from "./config";
 import {
-  DISPLAY_TIMEZONE,
   makeFormatters,
   type Formatters,
   type TimeFormatPreference,
@@ -32,6 +31,7 @@ import {
   primeMessages,
   type MessageBundle,
 } from "./load-locale";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 interface I18nContextValue {
   locale: Locale;
@@ -370,7 +370,7 @@ export function useDateFormatPreference(): DateFormatPreference {
 
 /**
  * The user's mirrored profile timezone, resolved to a renderable IANA zone:
- * valid mirror value → `DISPLAY_TIMEZONE` (Europe/Berlin). Reactive to
+ * valid mirror value → `DEFAULT_TIMEZONE` (Europe/Berlin). Reactive to
  * mirror changes (profile save, cross-tab, `/api/auth/me` re-fetch) via the
  * same localStorage-mirror pattern as the hour-cycle preference. SSR and the
  * pre-fetch state resolve Berlin — deliberately NO browser-timezone rung
@@ -386,7 +386,7 @@ export function useDisplayTimezone(): string {
     readStoredTimezone,
     () => "",
   );
-  return stored !== "" ? stored : DISPLAY_TIMEZONE;
+  return stored !== "" ? stored : DEFAULT_TIMEZONE;
 }
 
 /**
diff --git a/src/lib/insights/__tests__/biomarker-status.test.ts b/src/lib/insights/__tests__/biomarker-status.test.ts
index 50c586f06..4cd0bcc60 100644
--- a/src/lib/insights/__tests__/biomarker-status.test.ts
+++ b/src/lib/insights/__tests__/biomarker-status.test.ts
@@ -42,7 +42,7 @@ import { prisma } from "@/lib/db";
 import { runStatusCompletion } from "@/lib/insights/status-provider";
 import { hashInsightSnapshot } from "@/lib/insights/snapshot-hash";
 import { PROMPT_VERSION } from "@/lib/ai/prompts/base-system";
-import { toBerlinDayKey } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 import {
   generateBiomarkerStatus,
   biomarkerStatusScope,
@@ -142,7 +142,7 @@ describe("generateBiomarkerStatus — empty-data guard", () => {
 
 describe("generateBiomarkerStatus — cache read", () => {
   it("serves today's cached text without calling the provider", async () => {
-    const todayKey = toBerlinDayKey(new Date());
+    const todayKey = userDayKey(new Date(), "Europe/Berlin");
     vi.mocked(prisma.insightStatusCache.findUnique).mockResolvedValue(
       noteRow({
         dateKey: todayKey,
diff --git a/src/lib/insights/__tests__/bucket-series.test.ts b/src/lib/insights/__tests__/bucket-series.test.ts
index 2ddde27d9..136401419 100644
--- a/src/lib/insights/__tests__/bucket-series.test.ts
+++ b/src/lib/insights/__tests__/bucket-series.test.ts
@@ -3,7 +3,7 @@ import { describe, expect, it } from "vitest";
 import {
   applyPayloadBudget,
   bucketSeries,
-  dayOffsetToBerlinDayKey,
+  dayOffsetToDayKey,
 } from "../bucket-series";
 
 const dayMs = 24 * 60 * 60 * 1000;
@@ -75,7 +75,7 @@ describe("bucketSeries()", () => {
   });
 });
 
-describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
+describe("dayOffsetToDayKey() across DST boundaries", () => {
   // Europe/Berlin DST in 2024: spring-forward at 2024-03-31 02:00 → 03:00,
   // fall-back at 2024-10-27 03:00 → 02:00. Naive `now − dayOffset·86_400_000`
   // would crawl past those boundaries by an hour and silently land on the
@@ -85,8 +85,8 @@ describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
   it("produces today's Berlin date for dayOffset 0 around the spring-forward", () => {
     // 2024-03-31 11:30 Berlin (= 09:30 UTC, after the gap).
     const now = new Date("2024-03-31T09:30:00.000Z");
-    expect(dayOffsetToBerlinDayKey(now, 0)).toBe("2024-03-31");
-    expect(dayOffsetToBerlinDayKey(now, 1)).toBe("2024-03-30");
+    expect(dayOffsetToDayKey(now, 0)).toBe("2024-03-31");
+    expect(dayOffsetToDayKey(now, 1)).toBe("2024-03-30");
   });
 
   it("crossing the spring-forward DST boundary stays on the correct Berlin calendar day", () => {
@@ -96,10 +96,10 @@ describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
     // 11:30 CET → still 2024-03-30. That happens to look right here, so
     // the killer case is dayOffset 2 from a moment LATE on the day before
     // the gap.
-    expect(dayOffsetToBerlinDayKey(now, 0)).toBe("2024-04-02");
-    expect(dayOffsetToBerlinDayKey(now, 1)).toBe("2024-04-01");
-    expect(dayOffsetToBerlinDayKey(now, 2)).toBe("2024-03-31");
-    expect(dayOffsetToBerlinDayKey(now, 3)).toBe("2024-03-30");
+    expect(dayOffsetToDayKey(now, 0)).toBe("2024-04-02");
+    expect(dayOffsetToDayKey(now, 1)).toBe("2024-04-01");
+    expect(dayOffsetToDayKey(now, 2)).toBe("2024-03-31");
+    expect(dayOffsetToDayKey(now, 3)).toBe("2024-03-30");
   });
 
   it("crossing the spring-forward boundary from late-evening Berlin time does not slip a day", () => {
@@ -108,9 +108,9 @@ describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
     // 23:30 CET, which Intl formats as 2024-03-30. The CALENDAR-day
     // answer is 2024-03-31 — one Berlin calendar day earlier than today.
     const now = new Date("2024-03-31T22:30:00.000Z");
-    expect(dayOffsetToBerlinDayKey(now, 0)).toBe("2024-04-01");
-    expect(dayOffsetToBerlinDayKey(now, 1)).toBe("2024-03-31");
-    expect(dayOffsetToBerlinDayKey(now, 2)).toBe("2024-03-30");
+    expect(dayOffsetToDayKey(now, 0)).toBe("2024-04-01");
+    expect(dayOffsetToDayKey(now, 1)).toBe("2024-03-31");
+    expect(dayOffsetToDayKey(now, 2)).toBe("2024-03-30");
   });
 
   it("crossing the fall-back boundary from late-evening Berlin time does not slip a day", () => {
@@ -120,9 +120,9 @@ describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
     // 01:30 → 2024-10-27. That's actually correct by coincidence, so we
     // probe the dayOffset where the bias bites:
     const now = new Date("2024-10-27T23:30:00.000Z");
-    expect(dayOffsetToBerlinDayKey(now, 0)).toBe("2024-10-28");
-    expect(dayOffsetToBerlinDayKey(now, 1)).toBe("2024-10-27");
-    expect(dayOffsetToBerlinDayKey(now, 2)).toBe("2024-10-26");
+    expect(dayOffsetToDayKey(now, 0)).toBe("2024-10-28");
+    expect(dayOffsetToDayKey(now, 1)).toBe("2024-10-27");
+    expect(dayOffsetToDayKey(now, 2)).toBe("2024-10-26");
   });
 
   it("crossing the fall-back boundary the other direction (early-morning) stays put", () => {
@@ -131,9 +131,9 @@ describe("dayOffsetToBerlinDayKey() across DST boundaries", () => {
     // is what naive 24h subtraction gives because the day was 25 hours
     // long).
     const now = new Date("2024-10-27T01:30:00.000Z");
-    expect(dayOffsetToBerlinDayKey(now, 0)).toBe("2024-10-27");
-    expect(dayOffsetToBerlinDayKey(now, 1)).toBe("2024-10-26");
-    expect(dayOffsetToBerlinDayKey(now, 7)).toBe("2024-10-20");
+    expect(dayOffsetToDayKey(now, 0)).toBe("2024-10-27");
+    expect(dayOffsetToDayKey(now, 1)).toBe("2024-10-26");
+    expect(dayOffsetToDayKey(now, 7)).toBe("2024-10-20");
   });
 });
 
@@ -174,16 +174,14 @@ describe("bucketSeries() honours the per-user timezone (M-TZ3)", () => {
     expect(berlin).toEqual(def);
   });
 
-  it("dayOffsetToBerlinDayKey resolves the offset in the passed timezone", () => {
+  it("dayOffsetToDayKey resolves the offset in the passed timezone", () => {
     // `now` is 05-09 in LA but 05-10 in Berlin.
-    expect(dayOffsetToBerlinDayKey(reading, 0, "America/Los_Angeles")).toBe(
+    expect(dayOffsetToDayKey(reading, 0, "America/Los_Angeles")).toBe(
       "2026-05-09",
     );
     // Default (no tz) and explicit Berlin agree.
-    expect(dayOffsetToBerlinDayKey(reading, 0)).toBe("2026-05-10");
-    expect(dayOffsetToBerlinDayKey(reading, 0, "Europe/Berlin")).toBe(
-      "2026-05-10",
-    );
+    expect(dayOffsetToDayKey(reading, 0)).toBe("2026-05-10");
+    expect(dayOffsetToDayKey(reading, 0, "Europe/Berlin")).toBe("2026-05-10");
   });
 
   it("applyPayloadBudget threads the timezone through to the buckets", () => {
diff --git a/src/lib/insights/__tests__/graded-series-rollups.test.ts b/src/lib/insights/__tests__/graded-series-rollups.test.ts
index 1f386cfb8..7b79e904a 100644
--- a/src/lib/insights/__tests__/graded-series-rollups.test.ts
+++ b/src/lib/insights/__tests__/graded-series-rollups.test.ts
@@ -43,6 +43,8 @@ import {
 import { foldDayAggregates } from "@/lib/measurements/__tests__/fake-day-aggregates";
 import type { ReadDayAggregatesOptions } from "@/lib/measurements/day-aggregates";
 
+const TZ = "Europe/Berlin";
+
 const dayMs = 24 * 60 * 60 * 1000;
 
 /** Readings spanning `days` back from `now`, `perDay` per day. */
@@ -80,7 +82,7 @@ describe("buildGradedSeriesWithRollups — bounded reads (#1023)", () => {
       rows: [rollupRow(new Date("2025-09-01T00:00:00Z"), 79)],
     });
 
-    await buildGradedSeriesWithRollups("u1", "PULSE", now);
+    await buildGradedSeriesWithRollups("u1", "PULSE", now, TZ);
 
     expect(findMany).not.toHaveBeenCalled();
     const opts = readDayAggregates.mock.calls[0][0] as ReadDayAggregatesOptions;
@@ -99,7 +101,7 @@ describe("buildGradedSeriesWithRollups — bounded reads (#1023)", () => {
     serveFrom(points(800, now));
     readBestGranularityRollups.mockResolvedValue(null);
 
-    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now);
+    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now, TZ);
 
     expect(series.monthly.length).toBeGreaterThan(0);
     expect(series.yearly.length).toBeGreaterThan(0);
@@ -129,7 +131,7 @@ describe("buildGradedSeriesWithRollups — bounded reads (#1023)", () => {
             },
     );
 
-    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now);
+    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now, TZ);
 
     expect(series.monthly.length).toBeGreaterThan(0);
     expect(series.yearly.length).toBeGreaterThan(0);
@@ -148,7 +150,7 @@ describe("buildGradedSeriesWithRollups — bounded reads (#1023)", () => {
           : null,
     );
 
-    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now);
+    const series = await buildGradedSeriesWithRollups("u1", "WEIGHT", now, TZ);
 
     expect(series.monthly).toHaveLength(1);
     expect(series.yearly.length).toBeGreaterThan(0);
@@ -170,7 +172,7 @@ describe("buildGradedSeriesFromDayAggregates — parity with the raw fold", () =
       { measuredAt: new Date(now.getTime() - 21 * dayMs), value: 99 },
       { measuredAt: new Date(now.getTime() - 91 * dayMs), value: 41 },
     ];
-    const raw = buildGradedSeriesFromPoints(rows, now);
+    const raw = buildGradedSeriesFromPoints(rows, now, TZ);
     const agg = buildGradedSeriesFromDayAggregates(
       foldDayAggregates(rows, {
         since: new Date(0),
diff --git a/src/lib/insights/__tests__/graded-series.test.ts b/src/lib/insights/__tests__/graded-series.test.ts
index bf50c23af..edfb0f982 100644
--- a/src/lib/insights/__tests__/graded-series.test.ts
+++ b/src/lib/insights/__tests__/graded-series.test.ts
@@ -9,6 +9,8 @@ import {
   type SleepStageRow,
 } from "@/lib/analytics/sleep-night";
 
+const TZ = "Europe/Berlin";
+
 const dayMs = 24 * 60 * 60 * 1000;
 
 function dailyPoints(
@@ -34,7 +36,7 @@ describe("buildGradedSeriesFromPoints", () => {
   const now = new Date("2026-05-31T12:00:00Z");
 
   it("keeps the last ~14-21 days at daily granularity", () => {
-    const s = buildGradedSeriesFromPoints(dailyPoints(400, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(400, now), now, TZ);
     expect(s.recent.length).toBeGreaterThan(0);
     expect(s.recent.length).toBeLessThanOrEqual(21);
     // Each recent bucket carries daily aggregates.
@@ -51,7 +53,7 @@ describe("buildGradedSeriesFromPoints", () => {
   });
 
   it("folds the weeks after the recent window into ~8-10 ISO-week buckets", () => {
-    const s = buildGradedSeriesFromPoints(dailyPoints(400, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(400, now), now, TZ);
     expect(s.weekly.length).toBeGreaterThan(0);
     expect(s.weekly.length).toBeLessThanOrEqual(12);
     for (const w of s.weekly) {
@@ -64,7 +66,7 @@ describe("buildGradedSeriesFromPoints", () => {
   });
 
   it("folds older history into ~9-12 monthly buckets", () => {
-    const s = buildGradedSeriesFromPoints(dailyPoints(730, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(730, now), now, TZ);
     expect(s.monthly.length).toBeGreaterThan(0);
     expect(s.monthly.length).toBeLessThanOrEqual(14);
     for (const m of s.monthly) {
@@ -77,7 +79,7 @@ describe("buildGradedSeriesFromPoints", () => {
 
   it("folds multi-year history into yearly buckets with a trend slope", () => {
     // 3+ years so at least one year lands in the yearly bucket.
-    const s = buildGradedSeriesFromPoints(dailyPoints(1200, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(1200, now), now, TZ);
     expect(s.yearly.length).toBeGreaterThan(0);
     for (const y of s.yearly) {
       expect(y).toHaveProperty("year");
@@ -87,7 +89,7 @@ describe("buildGradedSeriesFromPoints", () => {
   });
 
   it("collapses a 2-year daily weigher into <= ~50 buckets, not 730", () => {
-    const s = buildGradedSeriesFromPoints(dailyPoints(730, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(730, now), now, TZ);
     // ~21 recent + ~10 weekly + ~12 monthly + ~2-3 yearly. Worst-case
     // boundary slack lands ~46; the point is the order-of-magnitude
     // collapse from 730 daily readings, not an exact bucket count.
@@ -95,12 +97,12 @@ describe("buildGradedSeriesFromPoints", () => {
   });
 
   it("returns empty buckets for an empty input", () => {
-    const s = buildGradedSeriesFromPoints([], now);
+    const s = buildGradedSeriesFromPoints([], now, TZ);
     expect(countBuckets(s)).toBe(0);
   });
 
   it("does not duplicate a day across recent and weekly", () => {
-    const s = buildGradedSeriesFromPoints(dailyPoints(120, now), now);
+    const s = buildGradedSeriesFromPoints(dailyPoints(120, now), now, TZ);
     // The most recent ISO week of the recent window must not also be a
     // weekly bucket — recent days are excluded from the weekly fold.
     const recentDates = new Set(s.recent.map((r) => r.date));
@@ -115,7 +117,7 @@ describe("buildGradedSeriesFromPoints", () => {
       { measuredAt: new Date(now.getTime() - 1000), value: 90 },
       { measuredAt: new Date(now.getTime() - 2000), value: 80 },
     ];
-    const s = buildGradedSeriesFromPoints(points, now);
+    const s = buildGradedSeriesFromPoints(points, now, TZ);
     expect(s.recent.length).toBe(1);
     expect(s.recent[0].min).toBe(70);
     expect(s.recent[0].max).toBe(90);
@@ -174,9 +176,27 @@ describe("buildGradedSeriesFromPoints", () => {
     const points = reconstructSleepNights(rows, "UTC")
       .filter((n) => n.asleepMinutes > 0)
       .map((n) => ({ measuredAt: n.measuredAt, value: n.asleepMinutes }));
-    const graded = buildGradedSeriesFromPoints(points, sleepNow);
+    const graded = buildGradedSeriesFromPoints(points, sleepNow, TZ);
     const recentMean = graded.recent.at(-1)?.mean ?? 0;
     expect(recentMean).toBe(480); // night total, not ~1218 (stage sum)
     expect(recentMean).toBeLessThan(960); // < 16 h — never impossible
   });
 });
+
+describe("buildGradedSeriesFromPoints — the user's calendar", () => {
+  it.each([
+    // 23:30 on 9 June in New York is already 10 June in UTC and Berlin.
+    ["America/New_York", "2026-06-10T03:30:00.000Z", "2026-06-09"],
+    // 08:00 on 10 June in Auckland and Tonga is still 9 June in UTC.
+    ["Pacific/Auckland", "2026-06-09T20:00:00.000Z", "2026-06-10"],
+    ["Pacific/Tongatapu", "2026-06-09T19:00:00.000Z", "2026-06-10"],
+  ])("keys a reading on its local day in %s", (tz, iso, day) => {
+    const now = new Date("2026-06-12T12:00:00.000Z");
+    const s = buildGradedSeriesFromPoints(
+      [{ measuredAt: new Date(iso), value: 70 }],
+      now,
+      tz,
+    );
+    expect(s.recent.map((r) => r.date)).toEqual([day]);
+  });
+});
diff --git a/src/lib/insights/__tests__/mood-aggregates.test.ts b/src/lib/insights/__tests__/mood-aggregates.test.ts
index 05a3bc8de..25c98de65 100644
--- a/src/lib/insights/__tests__/mood-aggregates.test.ts
+++ b/src/lib/insights/__tests__/mood-aggregates.test.ts
@@ -1006,7 +1006,12 @@ describe("computeTagMetricCrosstab", () => {
         meas(i, "ACTIVE_ENERGY_BURNED", 350 + (i % 2 === 0 ? 20 : -20)),
       );
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find(
       (r) => r.tag === "worked_out" && r.metricKey === "activeEnergy",
     );
@@ -1045,7 +1050,12 @@ describe("computeTagMetricCrosstab", () => {
         meas(i, "SLEEP_DURATION", 360 + (i % 2 === 0 ? 6 : -6)),
       );
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find((r) => r.metricKey === "sleepDuration");
     expect(row).toBeDefined();
     expect(row!.display).toBe("hours");
@@ -1079,7 +1089,12 @@ describe("computeTagMetricCrosstab", () => {
         meas(i - 1, "RECOVERY_SCORE", 80 + (i % 2 === 0 ? 3 : -3)),
       );
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find((r) => r.metricKey === "nextDayRecovery");
     expect(row).toBeDefined();
     expect(row!.mode).toBe("nextDay");
@@ -1134,7 +1149,12 @@ describe("computeTagMetricCrosstab", () => {
       62,
       54,
     );
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find(
       (r) => r.tag === "alcohol" && r.metricKey === "nextDayRestingHeartRate",
     );
@@ -1156,7 +1176,12 @@ describe("computeTagMetricCrosstab", () => {
       38,
       55,
     );
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find(
       (r) =>
         r.tag === "alcohol" && r.metricKey === "nextDayHeartRateVariability",
@@ -1183,6 +1208,7 @@ describe("computeTagMetricCrosstab", () => {
         { key, categoryKey: "health", labelKey, icon },
       );
       const rows = computeTagMetricCrosstab({
+        timeZone: "Europe/Berlin",
         entries,
         measurements,
         now: NOW,
@@ -1220,7 +1246,12 @@ describe("computeTagMetricCrosstab", () => {
       measurements.push(meas(i, "ACTIVE_ENERGY_BURNED", 350));
     }
     expect(CROSSTAB_MIN_PRESENT_DAYS).toBeGreaterThan(3);
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     expect(rows.find((r) => r.tag === "worked_out")).toBeUndefined();
   });
 
@@ -1235,7 +1266,12 @@ describe("computeTagMetricCrosstab", () => {
       entries.push(entry(i, 3, []));
       measurements.push(meas(i, "ACTIVE_ENERGY_BURNED", 350));
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     expect(rows).toEqual([]);
   });
 
@@ -1245,6 +1281,7 @@ describe("computeTagMetricCrosstab", () => {
       entries.push(structuredEntry(i, 4, i < 12 ? [WORKOUT] : []));
     }
     const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
       entries,
       measurements: [],
       now: NOW,
@@ -1301,7 +1338,12 @@ describe("computeTagMetricCrosstab", () => {
         ),
       );
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find(
       (r) => r.tag === "worked_out" && r.metricKey === "activeEnergy",
     );
@@ -1335,7 +1377,12 @@ describe("computeTagMetricCrosstab", () => {
         sourcedMeas(i, "SLEEP_DURATION", 360 + (i % 2 === 0 ? 6 : -6), "WHOOP"),
       );
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find((r) => r.metricKey === "sleepDuration");
     expect(row).toBeDefined();
     // 8 h, not 16 h — the Fitbit twin is dropped, not summed on top.
@@ -1370,7 +1417,12 @@ describe("computeTagMetricCrosstab", () => {
       measurements.push(sourcedMeas(i, "SLEEP_DURATION", 120 + j, "WHOOP"));
       measurements.push(sourcedMeas(i, "SLEEP_DURATION", 120 + j, "WHOOP"));
     }
-    const rows = computeTagMetricCrosstab({ entries, measurements, now: NOW });
+    const rows = computeTagMetricCrosstab({
+      timeZone: "Europe/Berlin",
+      entries,
+      measurements,
+      now: NOW,
+    });
     const row = rows.find((r) => r.metricKey === "sleepDuration");
     expect(row).toBeDefined();
     expect(row!.withAvg).toBeCloseTo(8, 1); // 3 × 160 = 480 min = 8 h
@@ -1478,6 +1530,7 @@ describe("computeFactorMetricCrosstab", () => {
       );
     }
     const rows = computeFactorMetricCrosstab({
+      timeZone: "Europe/Berlin",
       entries,
       measurements,
       now: NOW,
@@ -1517,6 +1570,7 @@ describe("computeFactorMetricCrosstab", () => {
       );
     }
     const rows = computeFactorMetricCrosstab({
+      timeZone: "Europe/Berlin",
       entries,
       measurements,
       now: NOW,
@@ -1543,6 +1597,7 @@ describe("computeFactorMetricCrosstab", () => {
       CROSSTAB_MIN_PRESENT_DAYS + CROSSTAB_MIN_ABSENT_DAYS,
     ).toBeGreaterThan(6);
     const rows = computeFactorMetricCrosstab({
+      timeZone: "Europe/Berlin",
       entries,
       measurements,
       now: NOW,
@@ -1558,7 +1613,12 @@ describe("computeFactorMetricCrosstab", () => {
       measurements.push(meas(i, "SLEEP_DURATION", 420));
     }
     expect(
-      computeFactorMetricCrosstab({ entries, measurements, now: NOW }),
+      computeFactorMetricCrosstab({
+        timeZone: "Europe/Berlin",
+        entries,
+        measurements,
+        now: NOW,
+      }),
     ).toEqual([]);
   });
 
diff --git a/src/lib/insights/blood-pressure-status.ts b/src/lib/insights/blood-pressure-status.ts
index a1c79ade4..b5bf3c6ec 100644
--- a/src/lib/insights/blood-pressure-status.ts
+++ b/src/lib/insights/blood-pressure-status.ts
@@ -33,7 +33,7 @@ import { sanitizeForPrompt } from "@/lib/insights/sanitize";
 import { getNoKeyBloodPressureStatusText } from "@/lib/insights/no-key-fallbacks";
 import {
   applyPayloadBudget,
-  dayOffsetToBerlinDayKey,
+  dayOffsetToDayKey,
   type DailyBucket,
 } from "@/lib/insights/bucket-series";
 import {
@@ -67,6 +67,7 @@ import { annotate } from "@/lib/logging/context";
 import { resolveUserTimezone, userDayKey } from "@/lib/tz/resolver";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { TRACKED_INTAKE_WHERE } from "@/lib/medications/intake-tracking";
+import { canonicalDailyTimestamp } from "@/lib/measurements/consolidation-tz";
 
 /**
  * Cap on the embedded correlation / paired-daily arrays. The Pearson
@@ -78,7 +79,7 @@ const CORRELATION_PAIR_CAP = 30;
 /**
  * Pair two daily-bucket series on `dayOffset`. The synthesised `date`
  * field is anchored at the UTC midnight of the Berlin calendar day —
- * `dayOffsetToBerlinDayKey()` is the source of truth so DST boundaries
+ * `dayOffsetToDayKey()` is the source of truth so DST boundaries
  * don't slip the day-key by one. Each pair also carries `dayKey`
  * directly so callers can label points without re-formatting.
  */
@@ -94,10 +95,9 @@ function pairDailyBuckets(
     .map((entry) => {
       const b = mapB.get(entry.dayOffset);
       if (b == null) return null;
-      const dayKey = dayOffsetToBerlinDayKey(now, entry.dayOffset, tz);
-      // UTC midnight of the Berlin day — formatting this Date with
-      // `toBerlinDayKey()` is guaranteed DST-safe because the y-m-d
-      // fields below are the Berlin calendar day fields by construction.
+      const dayKey = dayOffsetToDayKey(now, entry.dayOffset, tz);
+      // UTC midnight of the user's day: an ordering handle only; `dayKey`
+      // is the day itself.
       const [y, m, d] = dayKey.split("-").map(Number);
       return {
         a: entry.value,
@@ -435,8 +435,8 @@ export async function prepareBloodPressureStatusForUser(
   }
 
   const continuityVsSystolicSeries = sysSeries.daily.map((point) => {
-    // DST-safe: dayOffsetToBerlinDayKey computes calendar days, not 24h ticks.
-    const dayKey = dayOffsetToBerlinDayKey(now, point.dayOffset, userTz);
+    // DST-safe: dayOffsetToDayKey computes calendar days, not 24h ticks.
+    const dayKey = dayOffsetToDayKey(now, point.dayOffset, userTz);
     const taken = takenByDay.get(dayKey) ?? 0;
     const continuityPct =
       expectedBpIntakesPerDay > 0
@@ -480,18 +480,22 @@ export async function prepareBloodPressureStatusForUser(
   const gradedFromDaily = (buckets: DailyBucket[]) =>
     buildGradedSeriesFromPoints(
       buckets.map((b) => ({
-        measuredAt: new Date(dayOffsetToBerlinDayKey(now, b.dayOffset, userTz)),
+        measuredAt: canonicalDailyTimestamp(
+          dayOffsetToDayKey(now, b.dayOffset, userTz),
+          userTz,
+        ),
         value: b.value,
       })),
       now,
+      userTz,
     );
   // The two BP channels source their recent / weekly slices from a
   // bounded raw read and their monthly / yearly tail from the MONTH /
   // YEAR rollup tier (full-history in-memory fallback on a cold-tier
   // coverage miss). Mood has no rollup tier, so it stays an in-memory fold.
   const [sysGraded, diaGraded] = await Promise.all([
-    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_SYS", now),
-    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_DIA", now),
+    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_SYS", now, userTz),
+    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_DIA", now, userTz),
   ]);
   const moodGraded = gradedFromDaily(moodSeries.daily);
 
diff --git a/src/lib/insights/bmi-status.ts b/src/lib/insights/bmi-status.ts
index 0ad6d4a93..9c9808185 100644
--- a/src/lib/insights/bmi-status.ts
+++ b/src/lib/insights/bmi-status.ts
@@ -256,6 +256,7 @@ export async function prepareBmiStatusForUser(
     userId,
     "WEIGHT",
     now,
+    userTz,
   );
   const bmiGraded = scaleGradedSeries(weightGraded, 1 / heightFactor, 2);
   const bmiSeries = {
diff --git a/src/lib/insights/bucket-series.ts b/src/lib/insights/bucket-series.ts
index 0b100524b..fa45092b9 100644
--- a/src/lib/insights/bucket-series.ts
+++ b/src/lib/insights/bucket-series.ts
@@ -96,7 +96,7 @@ function utcMidnightOfUserDay(date: Date, tz: string): number {
  * `tz` defaults to Berlin only so legacy callers keep working; per-user
  * surfaces pass the resolved `User.timezone`.
  */
-export function dayOffsetToBerlinDayKey(
+export function dayOffsetToDayKey(
   now: Date,
   dayOffset: number,
   tz: string = DEFAULT_TIMEZONE,
diff --git a/src/lib/insights/comprehensive-generate.ts b/src/lib/insights/comprehensive-generate.ts
index 8d4d2ae40..0009faab5 100644
--- a/src/lib/insights/comprehensive-generate.ts
+++ b/src/lib/insights/comprehensive-generate.ts
@@ -886,6 +886,7 @@ export async function generateComprehensiveInsight(
     userId,
     dbUser?.displayName ?? null,
     locale,
+    await resolveUserTimezone(userId),
   );
   const systemPrompt = buildSystemPromptWithReferences(
     locale,
diff --git a/src/lib/insights/general-status.ts b/src/lib/insights/general-status.ts
index f506cae52..11f1fd5f1 100644
--- a/src/lib/insights/general-status.ts
+++ b/src/lib/insights/general-status.ts
@@ -218,7 +218,7 @@ export async function prepareGeneralStatusForUser(
 
       const series = applyPayloadBudget(records, { now, tz: userTz });
       dailyByType.set(type, series);
-      const graded = buildGradedSeriesFromPoints(records, now);
+      const graded = buildGradedSeriesFromPoints(records, now, userTz);
 
       return [
         [
@@ -298,7 +298,11 @@ export async function prepareGeneralStatusForUser(
     now,
     tz: userTz,
   });
-  const adherenceGraded = buildGradedSeriesFromPoints(adherenceRecords, now);
+  const adherenceGraded = buildGradedSeriesFromPoints(
+    adherenceRecords,
+    now,
+    userTz,
+  );
 
   // Fetch mood context (optional — for enrichment only). v1.4.28
   // FB-D2 — cap at 90 entries.
@@ -316,7 +320,7 @@ export async function prepareGeneralStatusForUser(
     value: entry.score,
   }));
   const moodSeries = applyPayloadBudget(moodRecords, { now, tz: userTz });
-  const moodGraded = buildGradedSeriesFromPoints(moodRecords, now);
+  const moodGraded = buildGradedSeriesFromPoints(moodRecords, now, userTz);
   const moodSummary = summarizeSeries(
     moodSeries.daily.map((bucket) => ({ value: bucket.value })),
   );
diff --git a/src/lib/insights/graded-series.ts b/src/lib/insights/graded-series.ts
index cf31d08f9..b67298080 100644
--- a/src/lib/insights/graded-series.ts
+++ b/src/lib/insights/graded-series.ts
@@ -57,7 +57,7 @@ import {
   readDayAggregates,
   type DayAggregateRow,
 } from "@/lib/measurements/day-aggregates";
-import { toBerlinYmd } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 
@@ -87,7 +87,7 @@ const MONTHLY_WINDOW_MS = WEEKLY_WINDOW_MS + MONTHLY_MONTHS * 30 * MS_PER_DAY;
 const COLD_FALLBACK_WINDOW_DAYS = 1095;
 
 export interface RecentDayBucket {
-  /** Berlin YYYY-MM-DD. */
+  /** YYYY-MM-DD in the user's zone. */
   date: string;
   min: number;
   max: number;
@@ -96,7 +96,7 @@ export interface RecentDayBucket {
 }
 
 export interface WeeklyBucket {
-  /** ISO week key like 2026-W19 (Berlin-anchored). */
+  /** ISO week key like 2026-W19 (the user's calendar). */
   weekISO: string;
   min: number;
   max: number;
@@ -105,7 +105,7 @@ export interface WeeklyBucket {
 }
 
 export interface MonthlyBucket {
-  /** Berlin YYYY-MM. */
+  /** YYYY-MM (the user's calendar; UTC for rollup months). */
   month: string;
   min: number;
   max: number;
@@ -114,7 +114,7 @@ export interface MonthlyBucket {
 }
 
 export interface YearlyBucket {
-  /** Berlin YYYY. */
+  /** YYYY (the user's calendar; UTC for rollup years). */
   year: string;
   min: number;
   max: number;
@@ -141,28 +141,13 @@ function round(value: number, digits = 2): number {
   return Math.round(value * factor) / factor;
 }
 
-function berlinDayKey(date: Date): string {
-  const { year, month, day } = toBerlinYmd(date);
-  return `${year}-${month}-${day}`;
-}
-
-function berlinMonthKey(date: Date): string {
-  const { year, month } = toBerlinYmd(date);
-  return `${year}-${month}`;
-}
-
-function berlinYearKey(date: Date): string {
-  return toBerlinYmd(date).year;
-}
-
-/**
- * ISO week key (Berlin-anchored). Mirrors the Coach's `isoWeekKey` but
- * fixed to the user's Berlin display day so the week label agrees with
- * the rest of the snapshot.
- */
-function berlinIsoWeekKey(date: Date): string {
-  const { year, month, day } = toBerlinYmd(date);
-  return isoWeekKeyOfYmd(year, month, day);
+/** The `YYYY-MM-DD` day `date` falls on in `timeZone`, as its parts. */
+function localYmd(
+  date: Date,
+  timeZone: string,
+): { year: string; month: string; day: string } {
+  const [year, month, day] = userDayKey(date, timeZone).split("-");
+  return { year, month, day };
 }
 
 /** ISO week key of a calendar date given as `YYYY`, `MM`, `DD`. */
@@ -228,6 +213,7 @@ function leastSquaresSlope(values: number[]): number | null {
 export function buildGradedSeriesFromPoints(
   points: Point[],
   now: Date,
+  timeZone: string,
 ): GradedSeries {
   const nowMs = now.getTime();
 
@@ -242,14 +228,16 @@ export function buildGradedSeriesFromPoints(
     const ageMs = nowMs - p.measuredAt.getTime();
     if (ageMs < 0) continue; // future reading — ignore
 
+    // Every bucket is a period of the user's own calendar.
+    const { year, month, day } = localYmd(p.measuredAt, timeZone);
     if (ageMs < RECENT_WINDOW_MS) {
-      foldInto(recentAgg, berlinDayKey(p.measuredAt), p.value);
+      foldInto(recentAgg, `${year}-${month}-${day}`, p.value);
     } else if (ageMs < WEEKLY_WINDOW_MS) {
-      foldInto(weeklyAgg, berlinIsoWeekKey(p.measuredAt), p.value);
+      foldInto(weeklyAgg, isoWeekKeyOfYmd(year, month, day), p.value);
     } else if (ageMs < MONTHLY_WINDOW_MS) {
-      foldInto(monthlyAgg, berlinMonthKey(p.measuredAt), p.value);
+      foldInto(monthlyAgg, `${year}-${month}`, p.value);
     } else {
-      const yk = berlinYearKey(p.measuredAt);
+      const yk = year;
       foldInto(yearlyAgg, yk, p.value);
       const list = yearlyValues.get(yk);
       if (list) list.push(p.value);
@@ -336,7 +324,7 @@ function gradedSegmentStarts(now: Date): Date[] {
  *
  * Replaces the raw `findMany` over the window (#1023): a watch that streams
  * heart rate every few seconds put a million rows into that read, which is
- * what exhausted the worker's heap. The rows here are one per Berlin day per
+ * what exhausted the worker's heap. The rows here are one per local day per
  * age segment, and their sum / count / min / max are exact, so the recent,
  * weekly and monthly slices come out identical to folding the raw rows.
  *
@@ -349,6 +337,7 @@ async function readGradedDayAggregates(
   type: MeasurementType,
   since: Date,
   now: Date,
+  timeZone: string,
 ): Promise<DayAggregateRow[]> {
   return readDayAggregates({
     userId,
@@ -356,7 +345,7 @@ async function readGradedDayAggregates(
     since,
     // Future-dated readings were skipped by the in-memory fold.
     until: now,
-    timeZone: "Europe/Berlin",
+    timeZone,
     // `readDayAggregates` puts a row at segment k when it is older than k
     // starts (`measured_at < start`). The in-memory fold treats an age of
     // exactly the window as the older bucket, so shift each start by 1 ms.
@@ -435,10 +424,12 @@ export function buildGradedSeriesFromDayAggregates(
 /**
  * Project a rollup bucket row to a monthly/yearly graded bucket. The
  * tier already carries min/max/mean/slope per bucket — no JS folding.
+ * Rollup buckets are UTC months and years, so the label is read off the
+ * bucket start in UTC: read in a zone west of UTC it named the month before.
  */
 function rollupMonthly(rows: RollupBucketRow[]): MonthlyBucket[] {
   return rows.map((r) => ({
-    month: berlinMonthKey(r.bucketStart),
+    month: r.bucketStart.toISOString().slice(0, 7),
     min: round(r.minValue),
     max: round(r.maxValue),
     mean: round(r.mean),
@@ -448,7 +439,7 @@ function rollupMonthly(rows: RollupBucketRow[]): MonthlyBucket[] {
 
 function rollupYearly(rows: RollupBucketRow[]): YearlyBucket[] {
   return rows.map((r) => ({
-    year: berlinYearKey(r.bucketStart),
+    year: r.bucketStart.toISOString().slice(0, 4),
     min: round(r.minValue),
     max: round(r.maxValue),
     mean: round(r.mean),
@@ -485,10 +476,11 @@ export async function buildGradedSeriesWithRollups(
   userId: string,
   type: MeasurementType,
   now: Date,
+  timeZone: string,
 ): Promise<GradedSeries> {
   const since = new Date(now.getTime() - WEEKLY_WINDOW_MS);
   const recentGraded = buildGradedSeriesFromDayAggregates(
-    await readGradedDayAggregates(userId, type, since, now),
+    await readGradedDayAggregates(userId, type, since, now, timeZone),
   );
 
   // v1.11.2 — load the source-priority blob once and thread it into both
@@ -548,7 +540,7 @@ export async function buildGradedSeriesWithRollups(
       now.getTime() - COLD_FALLBACK_WINDOW_DAYS * MS_PER_DAY,
     );
     const fullGraded = buildGradedSeriesFromDayAggregates(
-      await readGradedDayAggregates(userId, type, fallbackSince, now),
+      await readGradedDayAggregates(userId, type, fallbackSince, now, timeZone),
     );
     if (!monthlyCovered) monthly = fullGraded.monthly;
     if (!yearlyCovered) yearly = fullGraded.yearly;
diff --git a/src/lib/insights/medication-compliance-status.ts b/src/lib/insights/medication-compliance-status.ts
index 9971d3417..9008c0435 100644
--- a/src/lib/insights/medication-compliance-status.ts
+++ b/src/lib/insights/medication-compliance-status.ts
@@ -310,7 +310,7 @@ export async function prepareMedicationComplianceStatusForUser(
       now,
       tz: userTz,
     });
-    const dailySeries = buildGradedSeriesFromPoints(perDayRecords, now);
+    const dailySeries = buildGradedSeriesFromPoints(perDayRecords, now, userTz);
     const latestDay = dailyBudgeted.daily[0] ?? null;
 
     // v1.18.10 (HIGH-4) — the finished recent-vs-baseline adherence comparison
diff --git a/src/lib/insights/metric-correlation-context.ts b/src/lib/insights/metric-correlation-context.ts
index 811e8d452..56ac235f6 100644
--- a/src/lib/insights/metric-correlation-context.ts
+++ b/src/lib/insights/metric-correlation-context.ts
@@ -41,6 +41,7 @@ import {
   syncAcceptedPatterns,
 } from "@/lib/insights/correlation-patterns";
 import type { RelevantCorrelation } from "@/lib/insights/assessment-context";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 /** Trailing window for the discovery scan — mirrors the route. */
@@ -92,7 +93,7 @@ export async function getRelevantCorrelationsForMetric(
       where: { id: userId },
       select: { timezone: true },
     });
-    const tz = profile?.timezone ?? "Europe/Berlin";
+    const tz = profile?.timezone ?? DEFAULT_TIMEZONE;
     const since = new Date(Date.now() - WINDOW_DAYS * MS_PER_DAY);
 
     // The channel set is the shared one — every family the correlations page
diff --git a/src/lib/insights/metric-status.ts b/src/lib/insights/metric-status.ts
index c165d9828..a7d922cf3 100644
--- a/src/lib/insights/metric-status.ts
+++ b/src/lib/insights/metric-status.ts
@@ -308,11 +308,12 @@ export async function generateMetricStatus(args: {
   // straight from the deduped per-night TIME-ASLEEP `points` instead, so every
   // recent/weekly/monthly/yearly bucket is a night total, not a stage sum.
   const graded = isSleep
-    ? buildGradedSeriesFromPoints(points, now)
+    ? buildGradedSeriesFromPoints(points, now, userTz)
     : await buildGradedSeriesWithRollups(
         args.userId,
         meta.measurementType,
         now,
+        userTz,
       );
   const summary = summarizeSeries(
     series.daily.map((bucket) => ({ value: bucket.value })),
diff --git a/src/lib/insights/mood-aggregates.ts b/src/lib/insights/mood-aggregates.ts
index a6e634cd9..70dff13dd 100644
--- a/src/lib/insights/mood-aggregates.ts
+++ b/src/lib/insights/mood-aggregates.ts
@@ -27,7 +27,7 @@ import {
 import { MIN_PAIRED_N } from "@/lib/insights/correlations";
 import {
   applyPayloadBudget,
-  dayOffsetToBerlinDayKey,
+  dayOffsetToDayKey,
   type DailyBucket,
 } from "@/lib/insights/bucket-series";
 import { round, summarizeSeries } from "@/lib/insights/status-shared";
@@ -294,7 +294,7 @@ export const MOOD_ORANGE_MAX = 3.5;
 /**
  * Pair two daily-bucket series on `dayOffset`. The synthesised `date`
  * field is anchored at the UTC midnight of the Berlin calendar day —
- * `dayOffsetToBerlinDayKey()` is the source of truth so DST boundaries
+ * `dayOffsetToDayKey()` is the source of truth so DST boundaries
  * don't slip the day-key by one. Each pair also carries `dayKey`
  * directly so callers can label points without re-formatting.
  */
@@ -310,7 +310,7 @@ export function pairDailyBuckets(
     .map((entry) => {
       const b = mapB.get(entry.dayOffset);
       if (b == null) return null;
-      const dayKey = dayOffsetToBerlinDayKey(now, entry.dayOffset, tz);
+      const dayKey = dayOffsetToDayKey(now, entry.dayOffset, tz);
       const [y, m, d] = dayKey.split("-").map(Number);
       return {
         a: entry.value,
@@ -626,12 +626,14 @@ export function computeMoodAggregates(args: {
     measurements,
     now,
     userPriorityJson,
+    timeZone: tz,
   });
   const factorCrosstab = computeFactorMetricCrosstab({
     entries,
     measurements,
     now,
     userPriorityJson,
+    timeZone: tz,
   });
 
   // v1.38 — every context value against the day's pleasantness, one BH-FDR
@@ -682,7 +684,7 @@ export function computeMoodAggregates(args: {
         a4: entry.a4 ?? null,
         a5: entry.a5 ?? null,
       })),
-      dayOffsetToBerlinDayKey(now, 0, tz),
+      dayOffsetToDayKey(now, 0, tz),
     ),
     weekday,
     timeOfDay,
diff --git a/src/lib/insights/mood-crosstab.ts b/src/lib/insights/mood-crosstab.ts
index 181754cad..8e42fc9ff 100644
--- a/src/lib/insights/mood-crosstab.ts
+++ b/src/lib/insights/mood-crosstab.ts
@@ -17,7 +17,7 @@ import { welchTTest } from "@/lib/insights/correlations";
 import { round } from "@/lib/insights/status-shared";
 import { metricKeyForType } from "@/lib/measurements/cumulative-day-sum";
 import { MS_PER_DAY } from "@/lib/time-constants";
-import { toBerlinYmd } from "@/lib/tz/resolver";
+import { userDayKey } from "@/lib/tz/format";
 import type {
   CrossMetricMeasurement,
   MoodAggregateEntry,
@@ -181,14 +181,9 @@ const CROSSTAB_SUM_TYPES = new Set<string>([
   "ACTIVITY_STEPS",
 ]);
 
-/** Berlin-calendar day key (`YYYY-MM-DD`) for a row's `measuredAt`. */
-function berlinDayKey(measuredAt: Date): string {
-  const { year, month, day } = toBerlinYmd(measuredAt);
-  return `${year}-${month}-${day}`;
-}
-
 /**
- * Build a Berlin-day-keyed metric map with the right aggregation. Energy
+ * Build a metric map keyed by the user's local day, the calendar mood
+ * entries are dated on, with the right aggregation. Energy
  * and step totals are SUMMED per day (HealthKit `stats:` rows are already
  * one daily-total row, so the sum is the total either way); sleep duration
  * is SUMMED across per-stage rows to get the night total; everything else
@@ -198,7 +193,7 @@ function berlinDayKey(measuredAt: Date): string {
  * Cross-source de-dup: before bucketing, the rows for this metric run
  * through the SAME canonical-source picker the analytics steps/sleep path
  * uses (`pickCanonicalSourceRows`, keyed by `metricKeyForType` + the
- * Berlin day key). Without it, the moment two sources report the same day
+ * local day key). Without it, the moment two sources report the same day
  * (Fitbit + Apple steps, Fitbit + WHOOP sleep) the SUM channels would
  * double-count and bias the Welch delta. The picker collapses each day to
  * one source (and one device-type within it), so the sum reflects one
@@ -211,7 +206,10 @@ export function metricDayMap(
   measurements: CrossMetricMeasurement[],
   type: string,
   userPriorityJson: unknown,
+  /** The user's zone: days are the user's own calendar days. */
+  timeZone: string,
 ): Map<string, number> {
+  const dayKeyOf = (d: Date) => userDayKey(d, timeZone);
   const summed = CROSSTAB_SUM_TYPES.has(type);
 
   const typeRows = measurements.filter((m) => m.type === type);
@@ -235,13 +233,13 @@ export function metricDayMap(
         })),
         metricKey,
         userPriorityJson,
-        berlinDayKey,
+        dayKeyOf,
       ).canonicalRows
     : typeRows;
 
   const byDay = new Map<string, { sum: number; count: number }>();
   for (const m of canonicalRows) {
-    const key = berlinDayKey(m.measuredAt);
+    const key = dayKeyOf(m.measuredAt);
     const cur = byDay.get(key) ?? { sum: 0, count: 0 };
     cur.sum += m.value;
     cur.count += m.count ?? 1;
@@ -297,6 +295,8 @@ export function computeTagMetricCrosstab(args: {
    * test default) resolves to the default ladders.
    */
   userPriorityJson?: unknown;
+  /** The user's zone, the calendar the metric days are cut on. */
+  timeZone: string;
 }): TagMetricCrosstabRow[] {
   const { entries, measurements, now } = args;
   const windowDays = args.windowDays ?? 365;
@@ -319,7 +319,12 @@ export function computeTagMetricCrosstab(args: {
   for (const [metricKey, cfg] of Object.entries(CROSSTAB_METRICS) as Array<
     [CrosstabMetricKey, (typeof CROSSTAB_METRICS)[CrosstabMetricKey]]
   >) {
-    const metricByDay = metricDayMap(measurements, cfg.type, userPriorityJson);
+    const metricByDay = metricDayMap(
+      measurements,
+      cfg.type,
+      userPriorityJson,
+      args.timeZone,
+    );
     if (metricByDay.size === 0) continue;
 
     for (const [tagKey, ref] of structuredMeta) {
@@ -609,6 +614,8 @@ export function computeFactorMetricCrosstab(args: {
   now: Date;
   windowDays?: number;
   userPriorityJson?: unknown;
+  /** The user's zone, the calendar the metric days are cut on. */
+  timeZone: string;
 }): FactorMetricCrosstabRow[] {
   const { entries, measurements, now } = args;
   const windowDays = args.windowDays ?? 365;
@@ -627,7 +634,12 @@ export function computeFactorMetricCrosstab(args: {
       (typeof FACTOR_CROSSTAB_METRICS)[FactorCrosstabMetricKey],
     ]
   >) {
-    const metricByDay = metricDayMap(measurements, cfg.type, userPriorityJson);
+    const metricByDay = metricDayMap(
+      measurements,
+      cfg.type,
+      userPriorityJson,
+      args.timeZone,
+    );
     if (metricByDay.size === 0) continue;
 
     for (const [factorKey, series] of factorSeries) {
diff --git a/src/lib/insights/mood-narratives.ts b/src/lib/insights/mood-narratives.ts
index ef8107136..a1fa24626 100644
--- a/src/lib/insights/mood-narratives.ts
+++ b/src/lib/insights/mood-narratives.ts
@@ -14,7 +14,7 @@
  * "no correlation" filler.
  */
 
-import { dayOffsetToBerlinDayKey } from "@/lib/insights/bucket-series";
+import { dayOffsetToDayKey } from "@/lib/insights/bucket-series";
 import { round } from "@/lib/insights/status-shared";
 import { MS_PER_DAY } from "@/lib/time-constants";
 import { DEFAULT_TIMEZONE, userDayKey } from "@/lib/tz/format";
@@ -120,7 +120,7 @@ function weekdayOfOffset(
   dayOffset: number,
   tz: string = DEFAULT_TIMEZONE,
 ): number {
-  const dayKey = dayOffsetToBerlinDayKey(now, dayOffset, tz);
+  const dayKey = dayOffsetToDayKey(now, dayOffset, tz);
   const d = new Date(dayKey + "T00:00:00Z");
   return (d.getUTCDay() + 6) % 7;
 }
diff --git a/src/lib/insights/mood-patterns.ts b/src/lib/insights/mood-patterns.ts
index 86351aacf..7dc5fb218 100644
--- a/src/lib/insights/mood-patterns.ts
+++ b/src/lib/insights/mood-patterns.ts
@@ -10,7 +10,7 @@
  * `fetchMoodAggregates`.
  */
 
-import { dayOffsetToBerlinDayKey } from "@/lib/insights/bucket-series";
+import { dayOffsetToDayKey } from "@/lib/insights/bucket-series";
 import {
   MOOD_GREEN_MAX,
   MOOD_GREEN_MIN,
@@ -87,7 +87,7 @@ export function computeWeekdayAverages(
 ): WeekdayRow[] {
   const sums = new Map<number, { sum: number; count: number }>();
   for (const bucket of daily) {
-    const dayKey = dayOffsetToBerlinDayKey(now, bucket.dayOffset, tz);
+    const dayKey = dayOffsetToDayKey(now, bucket.dayOffset, tz);
     const d = new Date(dayKey + "T00:00:00Z");
     const weekday = (d.getUTCDay() + 6) % 7; // Monday = 0
     const cur = sums.get(weekday) ?? { sum: 0, count: 0 };
diff --git a/src/lib/insights/mood-status.ts b/src/lib/insights/mood-status.ts
index 13d11a647..a827b1dc9 100644
--- a/src/lib/insights/mood-status.ts
+++ b/src/lib/insights/mood-status.ts
@@ -305,7 +305,7 @@ export async function prepareMoodStatusForUser(
   // `applyPayloadBudget` daily buckets drive the in-target %, latest,
   // and correlations; the compact graded series reaches the prompt.
   const moodSeries = applyPayloadBudget(moodPoints, { now, tz: userTz });
-  const moodGraded = buildGradedSeriesFromPoints(moodPoints, now);
+  const moodGraded = buildGradedSeriesFromPoints(moodPoints, now, userTz);
   const moodSummary = summarizeSeries(
     moodSeries.daily.map((bucket) => ({ value: bucket.value })),
   );
@@ -490,6 +490,7 @@ export async function prepareMoodStatusForUser(
     measurements,
     now,
     userPriorityJson,
+    timeZone: userTz,
   });
 
   // v1.8.6 — the same threshold-gated narrative feed the user sees on
diff --git a/src/lib/insights/narrative/period-narrative-generate.ts b/src/lib/insights/narrative/period-narrative-generate.ts
index bb6cfc903..0d50e9dc3 100644
--- a/src/lib/insights/narrative/period-narrative-generate.ts
+++ b/src/lib/insights/narrative/period-narrative-generate.ts
@@ -53,6 +53,7 @@ import {
   validateNarrativeText,
   buildNarrativeCorrection,
 } from "@/lib/insights/narrative/narrative-grounding";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Stable identifier for the narrative prompt revision. Bumped whenever the
@@ -347,7 +348,7 @@ export async function generatePeriodNarrative(
     where: { id: userId },
     select: { timezone: true },
   });
-  const tz = profile?.timezone ?? "Europe/Berlin";
+  const tz = profile?.timezone ?? DEFAULT_TIMEZONE;
 
   const completion = await runCompletion({
     userId,
diff --git a/src/lib/insights/narrative/period-narrative.ts b/src/lib/insights/narrative/period-narrative.ts
index abe9d5c57..c5cab2167 100644
--- a/src/lib/insights/narrative/period-narrative.ts
+++ b/src/lib/insights/narrative/period-narrative.ts
@@ -55,6 +55,7 @@ import { buildBaselineBand, median } from "@/lib/insights/derived/baseline";
 import { VITALS_BASELINE_TYPES } from "@/lib/insights/derived/registry";
 import { assembleDiscoveryMatrix } from "@/lib/insights/discovery-matrix";
 import { resolveModuleMap } from "@/lib/modules/gate";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 const MS_PER_DAY = 24 * 60 * 60 * 1000;
 
@@ -480,7 +481,7 @@ export async function buildPeriodNarrativeContext(
     where: { id: userId },
     select: { timezone: true },
   });
-  const tz = profile?.timezone ?? "Europe/Berlin";
+  const tz = profile?.timezone ?? DEFAULT_TIMEZONE;
 
   const currentFrom = tzDayKey(
     new Date(now.getTime() - periodDays * MS_PER_DAY),
diff --git a/src/lib/insights/pulse-status.ts b/src/lib/insights/pulse-status.ts
index a2efb912f..084e17aed 100644
--- a/src/lib/insights/pulse-status.ts
+++ b/src/lib/insights/pulse-status.ts
@@ -233,7 +233,12 @@ export async function preparePulseStatusForUser(
   // Primary metric: recent / weekly fold from the bounded raw read, the
   // monthly / yearly tail comes from the MONTH / YEAR rollup tier (with
   // a full-history in-memory fallback on a cold-tier coverage miss).
-  const pulseGraded = await buildGradedSeriesWithRollups(userId, "PULSE", now);
+  const pulseGraded = await buildGradedSeriesWithRollups(
+    userId,
+    "PULSE",
+    now,
+    userTz,
+  );
   const pulseSummary = summarizeSeries(
     pulseSeries.daily.map((bucket) => ({ value: bucket.value })),
   );
@@ -255,7 +260,7 @@ export async function preparePulseStatusForUser(
     value: entry.score,
   }));
   const moodSeries = applyPayloadBudget(moodPoints, { now, tz: userTz });
-  const moodGraded = buildGradedSeriesFromPoints(moodPoints, now);
+  const moodGraded = buildGradedSeriesFromPoints(moodPoints, now, userTz);
   const moodSummary = summarizeSeries(
     moodSeries.daily.map((bucket) => ({ value: bucket.value })),
   );
diff --git a/src/lib/insights/weight-status.ts b/src/lib/insights/weight-status.ts
index 3486b75fe..c8afeeac9 100644
--- a/src/lib/insights/weight-status.ts
+++ b/src/lib/insights/weight-status.ts
@@ -24,7 +24,7 @@ import {
 import { getRelevantCorrelationsForMetric } from "@/lib/insights/metric-correlation-context";
 import {
   applyPayloadBudget,
-  dayOffsetToBerlinDayKey,
+  dayOffsetToDayKey,
   type DailyBucket,
 } from "@/lib/insights/bucket-series";
 import {
@@ -59,6 +59,7 @@ import {
 import { annotate } from "@/lib/logging/context";
 import { resolveUserTimezone, userDayKey } from "@/lib/tz/resolver";
 import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import { canonicalDailyTimestamp } from "@/lib/measurements/consolidation-tz";
 
 /**
  * Cap on the embedded correlation pair arrays. The Pearson coefficient
@@ -70,7 +71,7 @@ const CORRELATION_PAIR_CAP = 30;
 /**
  * Pair two daily-bucket series on `dayOffset`. The synthesised `date`
  * field is anchored at the UTC midnight of the Berlin calendar day —
- * `dayOffsetToBerlinDayKey()` is the source of truth so DST boundaries
+ * `dayOffsetToDayKey()` is the source of truth so DST boundaries
  * don't slip the day-key by one. Each pair also carries `dayKey`
  * directly so callers can label points without re-formatting.
  */
@@ -86,7 +87,7 @@ function pairDailyBuckets(
     .map((entry) => {
       const b = mapB.get(entry.dayOffset);
       if (b == null) return null;
-      const dayKey = dayOffsetToBerlinDayKey(now, entry.dayOffset, tz);
+      const dayKey = dayOffsetToDayKey(now, entry.dayOffset, tz);
       const [y, m, d] = dayKey.split("-").map(Number);
       return {
         a: entry.value,
@@ -370,16 +371,20 @@ export async function prepareWeightStatusForUser(
   // on a cold-tier coverage miss). Mood has no rollup tier, so it stays
   // an in-memory fold.
   const [weightGraded, sysGraded, diaGraded] = await Promise.all([
-    buildGradedSeriesWithRollups(userId, "WEIGHT", now),
-    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_SYS", now),
-    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_DIA", now),
+    buildGradedSeriesWithRollups(userId, "WEIGHT", now, userTz),
+    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_SYS", now, userTz),
+    buildGradedSeriesWithRollups(userId, "BLOOD_PRESSURE_DIA", now, userTz),
   ]);
   const moodGraded = buildGradedSeriesFromPoints(
     moodSeries.daily.map((b) => ({
-      measuredAt: new Date(dayOffsetToBerlinDayKey(now, b.dayOffset, userTz)),
+      measuredAt: canonicalDailyTimestamp(
+        dayOffsetToDayKey(now, b.dayOffset, userTz),
+        userTz,
+      ),
       value: b.value,
     })),
     now,
+    userTz,
   );
 
   const weightVsSystolicPairs = pairDailyBuckets(
@@ -408,7 +413,7 @@ export async function prepareWeightStatusForUser(
   // pairDailyBuckets — derive dayOffset from the offsets in sysSeries.
   const sysOffsetByDay = new Map(
     sysSeries.daily.map((bucket) => {
-      const dayKey = dayOffsetToBerlinDayKey(now, bucket.dayOffset, userTz);
+      const dayKey = dayOffsetToDayKey(now, bucket.dayOffset, userTz);
       return [dayKey, bucket.dayOffset];
     }),
   );
@@ -433,7 +438,7 @@ export async function prepareWeightStatusForUser(
   const latestWeight = weightSeries.daily[0] ?? null;
   const previousWeight = weightSeries.daily[1] ?? null;
   const latestWeightDay = latestWeight
-    ? dayOffsetToBerlinDayKey(now, latestWeight.dayOffset, userTz)
+    ? dayOffsetToDayKey(now, latestWeight.dayOffset, userTz)
     : null;
   const sameDayBp = latestWeightDay
     ? (pairedSystolicDiastolic.find((entry) => entry.day === latestWeightDay) ??
diff --git a/src/lib/jobs/apple-health-import-worker.ts b/src/lib/jobs/apple-health-import-worker.ts
index 2875cddf2..4105262c0 100644
--- a/src/lib/jobs/apple-health-import-worker.ts
+++ b/src/lib/jobs/apple-health-import-worker.ts
@@ -36,6 +36,7 @@ import { importAppleHealthEcg } from "@/lib/apple-health/ecg-import";
 import { extractExportXml } from "@/lib/import/unzip-export-xml";
 import { getGlobalBoss } from "@/lib/jobs/boss-instance";
 import { invalidateStatusInsightsForTypes } from "@/lib/insights/status-invalidation";
+import { trackBackgroundTask } from "@/lib/logging/background-tasks";
 import { jobDone, jobFailed, type JobOutcome } from "@/lib/jobs/job-outcome";
 import {
   streamParseExportXml,
@@ -47,6 +48,7 @@ import {
   ROLLUP_FOLD_WINDOW_MS,
 } from "@/lib/rollups/measurement-rollups";
 import { withBackgroundEvent } from "@/lib/logging/background";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Queue + cron for the periodic orphan-ImportJob sweep. v1.32.1
@@ -372,7 +374,7 @@ export async function handleAppleHealthImport(
     const userTimezone =
       userRow?.timezone && userRow.timezone.length > 0
         ? userRow.timezone
-        : "Europe/Berlin";
+        : DEFAULT_TIMEZONE;
 
     // Phase 1: unpacking
     await writeProgress(prisma, importJobId, "unpacking", {
@@ -542,12 +544,15 @@ export async function handleAppleHealthImport(
       .filter(([, stat]) => stat.inserted + stat.updated > 0)
       .map(([type]) => type as MeasurementType);
     if (importedTypes.length > 0) {
-      invalidateStatusInsightsForTypes(userId, importedTypes).catch((err) => {
-        console.warn(
-          `[apple-health-import] status-insight invalidate failed for user ${userId}`,
-          err,
-        );
-      });
+      // Registered so the test suite can await the detached handle.
+      trackBackgroundTask(
+        invalidateStatusInsightsForTypes(userId, importedTypes).catch((err) => {
+          console.warn(
+            `[apple-health-import] status-insight invalidate failed for user ${userId}`,
+            err,
+          );
+        }),
+      );
     }
 
     // Best-effort cleanup. A failed unlink is not fatal — `/tmp` is
diff --git a/src/lib/jobs/cycle-reminder.ts b/src/lib/jobs/cycle-reminder.ts
index e43288439..86111ae78 100644
--- a/src/lib/jobs/cycle-reminder.ts
+++ b/src/lib/jobs/cycle-reminder.ts
@@ -48,6 +48,7 @@ import { dispatchNotification } from "@/lib/notifications/dispatcher";
 import type { EventType } from "@/lib/notifications/types";
 import { isCycleReminderClientManaged } from "@/lib/validations/notification-prefs";
 import { getEvent } from "@/lib/logging/context";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Local-time hour (0–23) at which the daily cycle reminder fires. 09:00 is
@@ -138,7 +139,7 @@ export function cycleReminderLocalDate(
   now: Date,
   timezone: string,
 ): string | null {
-  const parts = wallClockInTz(now, timezone || "Europe/Berlin");
+  const parts = wallClockInTz(now, timezone || DEFAULT_TIMEZONE);
   if (parts.hour !== CYCLE_REMINDER_LOCAL_HOUR) return null;
   return `${parts.year.toString().padStart(4, "0")}-${parts.month
     .toString()
@@ -386,7 +387,7 @@ export async function runCycleReminderTick(
         continue;
       }
 
-      const timezone = user.timezone || "Europe/Berlin";
+      const timezone = user.timezone || DEFAULT_TIMEZONE;
       const today = cycleReminderLocalDate(now, timezone);
       if (!today) {
         summary.skippedOutsideWindow += 1;
diff --git a/src/lib/jobs/daily-briefing.ts b/src/lib/jobs/daily-briefing.ts
index 5edc1a1d7..74b825bf5 100644
--- a/src/lib/jobs/daily-briefing.ts
+++ b/src/lib/jobs/daily-briefing.ts
@@ -28,6 +28,7 @@ import {
   maybeDispatchDailyBriefing,
   type DailyBriefingDispatchDeps,
 } from "@/lib/daily/daily-briefing-push";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export const DAILY_BRIEFING_QUEUE = "daily-briefing";
 /** Every 15 minutes; the per-user local-hour gate selects the fallback slot. */
@@ -94,7 +95,7 @@ export async function runDailyBriefingTick(
   for (const pref of prefs) {
     const userId = pref.channel?.userId;
     if (!userId) continue;
-    cohort.set(userId, pref.channel.user?.timezone || "Europe/Berlin");
+    cohort.set(userId, pref.channel.user?.timezone || DEFAULT_TIMEZONE);
   }
 
   for (const [userId, tz] of cohort) {
diff --git a/src/lib/jobs/mood-reminder.ts b/src/lib/jobs/mood-reminder.ts
index 24e170f07..a5ad24492 100644
--- a/src/lib/jobs/mood-reminder.ts
+++ b/src/lib/jobs/mood-reminder.ts
@@ -32,6 +32,7 @@ import {
   DEFAULT_MOOD_REMINDER_HOUR,
   resolveMoodReminderHour,
 } from "@/lib/validations/notification-prefs";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Default hour of day (in the user's local timezone) at which the mood
@@ -88,7 +89,7 @@ export function evaluateMoodReminderWindow(
     return { fire: false, localDate: null, localHour: -1 };
   }
   const targetHour = user.reminderHour ?? MOOD_REMINDER_LOCAL_HOUR;
-  const parts = wallClockInTz(now, user.timezone || "Europe/Berlin");
+  const parts = wallClockInTz(now, user.timezone || DEFAULT_TIMEZONE);
   const inWindow = parts.hour === targetHour;
   const isoDate = `${parts.year.toString().padStart(4, "0")}-${parts.month
     .toString()
diff --git a/src/lib/jobs/period-narrative-warm.ts b/src/lib/jobs/period-narrative-warm.ts
index 5272f619e..6ade00411 100644
--- a/src/lib/jobs/period-narrative-warm.ts
+++ b/src/lib/jobs/period-narrative-warm.ts
@@ -42,6 +42,7 @@ import {
   enqueueNarrativeWarm,
   type PeriodNarrativePayload,
 } from "@/lib/jobs/period-narrative-shared";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export { PERIOD_NARRATIVE_QUEUE, PERIOD_NARRATIVE_CRON, enqueueNarrativeWarm };
 export type { PeriodNarrativePayload };
@@ -74,7 +75,7 @@ export interface NarrativeWarmRunResult {
  */
 export function periodsForDay(
   now: Date,
-  tz = "Europe/Berlin",
+  tz = DEFAULT_TIMEZONE,
 ): NarrativePeriod[] {
   // Resolve the local weekday + day-of-month in the maintenance-window tz so
   // the boundary matches the user-facing calendar, not UTC.
diff --git a/src/lib/jobs/reminder/medication-reminder-check.ts b/src/lib/jobs/reminder/medication-reminder-check.ts
index fd46f6a15..c30121a1a 100644
--- a/src/lib/jobs/reminder/medication-reminder-check.ts
+++ b/src/lib/jobs/reminder/medication-reminder-check.ts
@@ -44,6 +44,7 @@ import {
   LIVE_ERA_REVISION_ARGS,
   liveEraStart as liveEraStartOf,
 } from "@/lib/medications/scheduling/live-era";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 export interface ReminderCheckPayload {
   triggeredAt: string;
@@ -198,7 +199,7 @@ export async function handleReminderCheck(
       let notificationsFailed = 0;
 
       for (const med of medications) {
-        const userTz = med.user.timezone || "Europe/Berlin";
+        const userTz = med.user.timezone || DEFAULT_TIMEZONE;
         const { start: todayStart, end: todayEnd } = getUserTodayBounds(
           now,
           userTz,
diff --git a/src/lib/jobs/reminder/registrar-shared.ts b/src/lib/jobs/reminder/registrar-shared.ts
index c05f1ea87..c46497fb8 100644
--- a/src/lib/jobs/reminder/registrar-shared.ts
+++ b/src/lib/jobs/reminder/registrar-shared.ts
@@ -28,6 +28,7 @@ import {
   type JobHandler,
   type JobWithMetadataHandler,
 } from "@/lib/jobs/run-job";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Cron schedule tuple: `[queueName, cronExpression, sendOptions?]`. The
@@ -223,7 +224,7 @@ export async function createAndSchedule(
       cron,
       {},
       {
-        tz: "Europe/Berlin",
+        tz: DEFAULT_TIMEZONE,
         ...(sendOptions ?? {}),
       },
     );
diff --git a/src/lib/mcp/tools.ts b/src/lib/mcp/tools.ts
index b0f772174..61ebb48cb 100644
--- a/src/lib/mcp/tools.ts
+++ b/src/lib/mcp/tools.ts
@@ -1518,7 +1518,7 @@ export const MCP_TOOLS: McpToolDefinition[] = [
         return { present: false };
       }
 
-      const userTz = user?.timezone || "Europe/Berlin";
+      const userTz = user?.timezone || DEFAULT_TIMEZONE;
 
       // Same feeder reads + horizon the medications list route / dashboard
       // builder use, so the open-overdue detection matches the in-app cards.
diff --git a/src/lib/mcp/writes.ts b/src/lib/mcp/writes.ts
index aa0047673..49e3365bd 100644
--- a/src/lib/mcp/writes.ts
+++ b/src/lib/mcp/writes.ts
@@ -40,7 +40,8 @@ import { afterMeasurementMutation } from "@/lib/rollups/after-measurement-mutati
 import { recomputeMoodBucketsForEntry } from "@/lib/rollups/mood-rollups";
 import { MOOD_ENUM_BY_SCORE } from "@/lib/mood/labels";
 import { getScoreForMood } from "@/lib/validations/mood";
-import { moodDateKey, DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { deriveA1 } from "@/lib/mood/level-a";
 import {
   classifyExternalId,
diff --git a/src/lib/measurements/consolidation-base.ts b/src/lib/measurements/consolidation-base.ts
index 43f9d576d..8c7a447f3 100644
--- a/src/lib/measurements/consolidation-base.ts
+++ b/src/lib/measurements/consolidation-base.ts
@@ -40,13 +40,13 @@ import {
   dayKeyForUserTz,
   type PerSampleRow,
 } from "./consolidation-tz";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 // Re-export the shared grace constant so the established
 // `consolidation-base` import surface stays unchanged for callers.
 export { CONSOLIDATION_GRACE_CUTOFF_HOURS };
 
 /** Fallback timezone for users with no `User.timezone` set. */
-const DEFAULT_TIMEZONE = "Europe/Berlin";
 
 /**
  * Options shared by every drain. Each drain re-exports its own alias of
diff --git a/src/lib/measurements/dense-intraday-retention.ts b/src/lib/measurements/dense-intraday-retention.ts
index 5c6772c3c..869010135 100644
--- a/src/lib/measurements/dense-intraday-retention.ts
+++ b/src/lib/measurements/dense-intraday-retention.ts
@@ -67,6 +67,7 @@ import { meanBucketValue } from "./consolidate-daily-mean";
 import { recomputeBucketsForMeasurement } from "@/lib/rollups/measurement-rollups";
 import { percentile } from "@/lib/insights/strain-score";
 import type { PerSampleRow } from "./drain-per-sample-cumulative";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * iOS#34 / #69 — heart-rate (PULSE) is the densest spot signal after
@@ -533,7 +534,7 @@ export async function runDenseIntradayRetention(
 
   // The base walks users serially; recordBucket needs the current user's tz
   // for the dry-run hourly fan-out estimate (writeDay never runs on dry-run).
-  let currentTz = "Europe/Berlin";
+  let currentTz = DEFAULT_TIMEZONE;
 
   const walk = await runConsolidation<MeasurementType>({
     prismaClient,
diff --git a/src/lib/mood/__tests__/date-key.test.ts b/src/lib/mood/__tests__/date-key.test.ts
index f815e54f4..3053f7547 100644
--- a/src/lib/mood/__tests__/date-key.test.ts
+++ b/src/lib/mood/__tests__/date-key.test.ts
@@ -11,7 +11,8 @@
  *     are fixing for non-Berlin users.
  */
 import { describe, expect, it } from "vitest";
-import { moodDateKey, effectiveMoodTz, DEFAULT_TIMEZONE } from "../date-key";
+import { moodDateKey, effectiveMoodTz } from "../date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 describe("moodDateKey", () => {
   it("formats a UTC instant as a Europe/Berlin YYYY-MM-DD by default", () => {
diff --git a/src/lib/mood/create-from-telegram.ts b/src/lib/mood/create-from-telegram.ts
index 3b5c4d3eb..e54e527eb 100644
--- a/src/lib/mood/create-from-telegram.ts
+++ b/src/lib/mood/create-from-telegram.ts
@@ -18,7 +18,8 @@ import { prisma as defaultPrisma } from "@/lib/db";
 import { MOOD_ENUM_BY_SCORE } from "@/lib/mood/labels";
 import { getScoreForMood } from "@/lib/validations/mood";
 import { deriveA1 } from "@/lib/mood/level-a";
-import { moodDateKey, DEFAULT_TIMEZONE } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import { encryptNote } from "@/lib/crypto/note-cipher";
 import { invalidateUserMood } from "@/lib/cache/invalidate";
 import { recomputeMoodBucketsForEntry } from "@/lib/rollups/mood-rollups";
diff --git a/src/lib/mood/date-key.ts b/src/lib/mood/date-key.ts
index 440561f0a..744ef9d58 100644
--- a/src/lib/mood/date-key.ts
+++ b/src/lib/mood/date-key.ts
@@ -28,9 +28,7 @@
  * legacy date string (`effectiveMoodTz`).
  */
 
-import { userDayKey } from "@/lib/tz/format";
-
-export const DEFAULT_TIMEZONE = "Europe/Berlin";
+import { userDayKey, DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Build the `YYYY-MM-DD` day-key for a mood entry given the user's
diff --git a/src/lib/mood/linked-context.ts b/src/lib/mood/linked-context.ts
index 318dcc6cd..a39910892 100644
--- a/src/lib/mood/linked-context.ts
+++ b/src/lib/mood/linked-context.ts
@@ -48,7 +48,8 @@ import { prisma } from "@/lib/db";
 import { isModuleEnabled } from "@/lib/modules/gate";
 import { moduleForMeasurementType } from "@/lib/modules/measurement-scope";
 import type { ModuleKey } from "@/lib/modules/registry";
-import { DEFAULT_TIMEZONE, moodDateKey } from "@/lib/mood/date-key";
+import { moodDateKey } from "@/lib/mood/date-key";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 import {
   reconstructSleepNights,
   type SleepStageRow,
diff --git a/src/lib/openapi/routes/mood.ts b/src/lib/openapi/routes/mood.ts
index 186331aa3..5e80702f8 100644
--- a/src/lib/openapi/routes/mood.ts
+++ b/src/lib/openapi/routes/mood.ts
@@ -60,6 +60,7 @@ import {
   stdResponses,
   updatedAtTokenField,
 } from "./shared";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 // ── Request schemas — annotated for spec emission ────────────────────
 //
@@ -458,7 +459,7 @@ const moodEntryDto = z
     date: z
       .string()
       .describe(
-        "YYYY-MM-DD, anchored to the row's `tz` (legacy rows with `tz: null` read as Europe/Berlin).",
+        `YYYY-MM-DD, anchored to the row's \`tz\` (legacy rows with \`tz: null\` read as ${DEFAULT_TIMEZONE}).`,
       ),
     mood: moodLevelEnum,
     score: z
@@ -501,7 +502,7 @@ const moodEntryDto = z
       .string()
       .nullable()
       .describe(
-        "IANA zone the `date` label is anchored to. Null on legacy rows (read as Europe/Berlin).",
+        `IANA zone the \`date\` label is anchored to. Null on legacy rows (read as ${DEFAULT_TIMEZONE}).`,
       ),
     syncedAt: z.iso.datetime({ offset: true }),
     createdAt: z.iso.datetime({ offset: true }),
diff --git a/src/lib/personal-records/pr-detection-worker.ts b/src/lib/personal-records/pr-detection-worker.ts
index 77739c6a5..ae7ef6f80 100644
--- a/src/lib/personal-records/pr-detection-worker.ts
+++ b/src/lib/personal-records/pr-detection-worker.ts
@@ -61,6 +61,7 @@ import {
   type MeasurementType,
   type PrismaClient,
 } from "@/generated/prisma/client";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /**
  * Minimum sample count per metric before the worker writes its first
@@ -245,7 +246,7 @@ export async function detectPersonalRecordsForUser(
   const tz =
     userRow?.timezone && userRow.timezone.length > 0
       ? userRow.timezone
-      : "Europe/Berlin";
+      : DEFAULT_TIMEZONE;
   const priorityJson = userRow?.sourcePriorityJson ?? null;
 
   // v1.34 — resolve the weight goal once per run: the user's own target band
diff --git a/src/lib/timezone-mirror.ts b/src/lib/timezone-mirror.ts
index e5a4b37a4..baddc7ab4 100644
--- a/src/lib/timezone-mirror.ts
+++ b/src/lib/timezone-mirror.ts
@@ -11,7 +11,7 @@
  *   2. the legacy helpers in `src/lib/format.ts` (plain functions, no React
  *      context) render the same zone as hook-based call sites.
  *
- * Fallback chain: valid mirrored profile zone → `DISPLAY_TIMEZONE`
+ * Fallback chain: valid mirrored profile zone → `DEFAULT_TIMEZONE`
  * (Europe/Berlin) inside `makeFormatters`. Deliberately NO browser-timezone
  * rung — every server-rendered artifact (doctor-report PDF, exports,
  * briefing) resolves profile → Berlin, and a browser rung would let the same
@@ -36,7 +36,7 @@ const CHANGE_EVENT = "healthlog:timezone-change";
 
 /**
  * Best-effort read of the mirrored zone. "" on SSR / no mirror / invalid
- * stored value — callers treat "" as "fall back to `DISPLAY_TIMEZONE`"
+ * stored value — callers treat "" as "fall back to `DEFAULT_TIMEZONE`"
  * (which `makeFormatters` does on its own for an empty `userTz`).
  */
 export function readStoredTimezone(): string {
diff --git a/src/lib/tz/format.ts b/src/lib/tz/format.ts
index 4277a48a0..8f026c41d 100644
--- a/src/lib/tz/format.ts
+++ b/src/lib/tz/format.ts
@@ -178,7 +178,7 @@ export type FormatInUserTzShape =
  *   - "datetime" — `2026-05-11 11:05` (24h, locale-independent for
  *     audit/PDF tables). Equivalent to the `formatters.dateTime`
  *     helper but with a per-call timezone instead of the global
- *     `DISPLAY_TIMEZONE`.
+ *     `DEFAULT_TIMEZONE`.
  *
  *   - "date" — `2026-05-11` (ISO date in the user's zone). Used as a
  *     stable day-bucket key for per-user aggregation.
diff --git a/src/lib/tz/resolver.ts b/src/lib/tz/resolver.ts
index 6f3034dc5..8224b07ec 100644
--- a/src/lib/tz/resolver.ts
+++ b/src/lib/tz/resolver.ts
@@ -2,7 +2,7 @@
  * v1.4.25 W7 — per-user timezone resolver (server-only).
  *
  * HealthLog historically pinned every display surface to the
- * `DISPLAY_TIMEZONE` constant ("Europe/Berlin"). The per-user
+ * `DEFAULT_TIMEZONE` constant ("Europe/Berlin"). The per-user
  * timezone feature (Option B in `.planning/feature-user-timezone.md`)
  * threads the user's stored zone through every surface that renders
  * a date or buckets by day — exports, charts, AI Coach snapshot,
@@ -51,54 +51,6 @@ export {
   hourInTz,
 } from "./format";
 
-const BERLIN_DAY_FORMATTER = new Intl.DateTimeFormat("en-US", {
-  timeZone: "Europe/Berlin",
-  year: "numeric",
-  month: "2-digit",
-  day: "2-digit",
-});
-
-/**
- * Format a Date as `YYYY-MM-DD` in `Europe/Berlin`. The seven Insight
- * status helpers (`bmi`, `blood-pressure`, `weight`, `pulse`, `mood`,
- * `medication-compliance`, `general`) all key their per-day cache on
- * this — the maintainer's primary tenant runs in Berlin and the cache
- * action row reads identically across helpers when the formatter lives
- * here.
- *
- * For per-user surfaces, prefer `userDayKey(date, tz)` from `./format`.
- * This helper exists for the cache-key path where the day-bucket is
- * intentionally Berlin-anchored regardless of the requesting user's
- * stored zone.
- */
-export function toBerlinDayKey(date: Date): string {
-  const { year, month, day } = toBerlinYmd(date);
-  return `${year}-${month}-${day}`;
-}
-
-/**
- * Same formatter, but returns the numeric `{year, month, day}` parts
- * instead of the reassembled string. Used by `bucket-series.ts` for
- * `Date.UTC(year, month - 1, day)` arithmetic where parsing the string
- * back would be wasted work.
- */
-export function toBerlinYmd(date: Date): {
-  year: string;
-  month: string;
-  day: string;
-} {
-  const parts = BERLIN_DAY_FORMATTER.formatToParts(date);
-  const year = parts.find((part) => part.type === "year")?.value;
-  const month = parts.find((part) => part.type === "month")?.value;
-  const day = parts.find((part) => part.type === "day")?.value;
-
-  if (!year || !month || !day) {
-    throw new Error("Could not derive Berlin day key");
-  }
-
-  return { year, month, day };
-}
-
 const TTL_MS = 60_000;
 
 interface CacheEntry {
diff --git a/src/lib/vaccinations/service.ts b/src/lib/vaccinations/service.ts
index 8d2212bd2..686151f1a 100644
--- a/src/lib/vaccinations/service.ts
+++ b/src/lib/vaccinations/service.ts
@@ -17,9 +17,9 @@ import {
   deriveSeries,
   type SeriesInputRecord,
 } from "@/lib/vaccinations/series";
+import { DEFAULT_TIMEZONE } from "@/lib/tz/format";
 
 /** Matches the visits service: the fallback when an account never set one. */
-const DEFAULT_TIMEZONE = "Europe/Berlin";
 
 /** The relations every vaccination response resolves. */
 export const VACCINATION_INCLUDE = {
diff --git a/tests/integration/cycle-crosstab-day-aggregates.test.ts b/tests/integration/cycle-crosstab-day-aggregates.test.ts
index cdbc2df2e..43a04ef05 100644
--- a/tests/integration/cycle-crosstab-day-aggregates.test.ts
+++ b/tests/integration/cycle-crosstab-day-aggregates.test.ts
@@ -101,8 +101,8 @@ describe("cycle crosstab day values (#1023)", () => {
     expect(aggRows.length).toBeLessThan(500);
 
     for (const type of PHASE_CROSSTAB_METRIC_TYPES) {
-      const expected = metricDayMap(raw, type, null);
-      const actual = metricDayMap(aggRows, type, null);
+      const expected = metricDayMap(raw, type, null, "Europe/Berlin");
+      const actual = metricDayMap(aggRows, type, null, "Europe/Berlin");
       expect([...actual.keys()].sort(), type).toEqual(
         [...expected.keys()].sort(),
       );
diff --git a/tests/integration/dense-read-bounded.test.ts b/tests/integration/dense-read-bounded.test.ts
index 009a566a6..6357877b7 100644
--- a/tests/integration/dense-read-bounded.test.ts
+++ b/tests/integration/dense-read-bounded.test.ts
@@ -113,7 +113,7 @@ afterAll(async () => {
 describe("pulse status over a dense stream (#1023)", () => {
   it("builds the graded series without materialising the stream", async () => {
     const graded = await measure(() =>
-      buildGradedSeriesWithRollups(DENSE_USER, "PULSE", NOW),
+      buildGradedSeriesWithRollups(DENSE_USER, "PULSE", NOW, "Europe/Berlin"),
     );
     const series = graded.result;
     // Every reading in the 91-day window is accounted for, one bucket per day