diff --git a/src/lib/analytics/__tests__/resting-pulse.test.ts b/src/lib/analytics/__tests__/resting-pulse.test.ts
index 666e6500f..af41b0e4d 100644
--- a/src/lib/analytics/__tests__/resting-pulse.test.ts
+++ b/src/lib/analytics/__tests__/resting-pulse.test.ts
@@ -12,11 +12,45 @@ import {
   resolveRestingPulseSeries,
   type PulseSample,
 } from "../resting-pulse";
+import { zonedWallClockToUtc } from "@/lib/tz/wall-clock";
 
-const day = (iso: string, value: number): PulseSample => ({
-  measuredAt: new Date(iso),
-  value,
-});
+/**
+ * The zone `deriveRestingProxyFromPulse` buckets in by default
+ * (`toBerlinDayKey`). Naming it here is the point: the fixtures used to
+ * pass offset-less ISO literals, which `new Date(...)` reads as HOST-local.
+ * That made every expectation mean something different on every developer
+ * machine, and — because a Berlin day and a UTC day coincide for samples in
+ * the middle of the day — swapping the implementation's day-key zone left
+ * the whole file green. Building each instant explicitly in Berlin lets the
+ * assertions actually pin the bucketing.
+ */
+const PROXY_DAY_ZONE = "Europe/Berlin";
+
+/**
+ * A pulse sample at the given `YYYY-MM-DDTHH:MM:SS` wall clock **in
+ * Berlin**, expressed as the UTC instant it denotes.
+ */
+const day = (wallClock: string, value: number): PulseSample => {
+  const m = wallClock.match(
+    /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})$/,
+  );
+  if (!m) throw new Error(`fixture wall clock must be local, got ${wallClock}`);
+  const [, y, mo, d, h, mi, sec] = m;
+  return {
+    measuredAt: zonedWallClockToUtc(
+      {
+        year: Number(y),
+        month: Number(mo),
+        day: Number(d),
+        hour: Number(h),
+        minute: Number(mi),
+        second: Number(sec),
+      },
+      PROXY_DAY_ZONE,
+    ),
+    value,
+  };
+};
 
 describe("deriveRestingProxyFromPulse", () => {
   it("excludes a workout burst — the daily proxy tracks the resting floor", () => {
@@ -154,3 +188,55 @@ describe("resolveRestingPulseSeries", () => {
     ).toEqual({ series: [], which: "none" });
   });
 });
+
+describe("deriveRestingProxyFromPulse — the day bucket is the PROFILE day", () => {
+  it("keeps after-midnight local samples on their own local day, not the UTC one", () => {
+    // Berlin is UTC+2 in June, so 00:20–01:00 local on 2026-06-02 is
+    // 22:20–23:00 UTC on 2026-06-01 — a DIFFERENT UTC calendar day. All
+    // six samples belong to one Berlin day and must produce ONE bucket.
+    // Under a UTC day-key they split 3/3 and produce two.
+    const samples = [
+      day("2026-06-02T00:20:00", 60),
+      day("2026-06-02T00:40:00", 62),
+      day("2026-06-02T01:00:00", 64),
+      day("2026-06-02T09:00:00", 66),
+      day("2026-06-02T15:00:00", 68),
+      day("2026-06-02T21:00:00", 70),
+    ];
+    const proxy = deriveRestingProxyFromPulse(samples);
+    expect(proxy).toHaveLength(1);
+  });
+
+  it("splits two local days that a UTC key would merge", () => {
+    // 23:10–23:50 Berlin on 2026-06-01 is 21:10–21:50 UTC the same day,
+    // while 00:10–00:50 Berlin on 2026-06-02 is 22:10–22:50 UTC on
+    // 2026-06-01. A UTC key merges all six into one bucket; the Berlin key
+    // must see two days.
+    const samples = [
+      day("2026-06-01T23:10:00", 60),
+      day("2026-06-01T23:30:00", 62),
+      day("2026-06-01T23:50:00", 64),
+      day("2026-06-02T00:10:00", 80),
+      day("2026-06-02T00:30:00", 82),
+      day("2026-06-02T00:50:00", 84),
+    ];
+    const proxy = deriveRestingProxyFromPulse(samples);
+    expect(proxy).toHaveLength(2);
+  });
+
+  it("honours an injected day-key function over the Berlin default", () => {
+    // The targets route threads `userDayKey(d, userTz)`. Same samples,
+    // different zone → a different bucketing. This is the seam the default
+    // hides, so it gets its own pin.
+    const samples = [
+      day("2026-06-02T00:20:00", 60),
+      day("2026-06-02T00:40:00", 62),
+      day("2026-06-02T01:00:00", 64),
+    ];
+    const utcKey = (d: Date) => d.toISOString().slice(0, 10);
+    // In UTC these three are 2026-06-01 22:20/22:40/23:00 → one UTC day.
+    expect(deriveRestingProxyFromPulse(samples, utcKey)).toHaveLength(1);
+    // And that UTC day is 06-01, whereas the Berlin bucket is 06-02.
+    expect(utcKey(samples[0].measuredAt)).toBe("2026-06-01");
+  });
+});
diff --git a/src/lib/analytics/compliance/__tests__/compliance.test.ts b/src/lib/analytics/compliance/__tests__/compliance.test.ts
index f14be4e05..b360f6218 100644
--- a/src/lib/analytics/compliance/__tests__/compliance.test.ts
+++ b/src/lib/analytics/compliance/__tests__/compliance.test.ts
@@ -5,8 +5,18 @@
  * the numbers now honour `daysOfWeek` and `intervalWeeks`. These
  * tests pin the contract for every cadence the production app
  * exercises: daily 1×/day, daily multi-dose, weekly Mondays-only,
- * bi-weekly, weekday-only multi-dose, and the DST-boundary day.
- * Closes #214.
+ * bi-weekly, and weekday-only multi-dose. Closes #214.
+ *
+ * The DST local-day walk is NOT pinned here. A "DST boundary" case used
+ * to sit in this file passing no timezone, so the slot builder took the
+ * legacy UTC path where the date carries no transition — it asserted
+ * `totalExpected === 7`, which holds in every zone and therefore could
+ * never see a double-emit. Threading a real zone through it does not
+ * help: the assertion stays true in Berlin, UTC, UTC+14 and UTC−11
+ * alike. The property it claimed lives in
+ * `src/lib/medications/scheduling/__tests__/compliance.test.ts`, which
+ * walks `buildCadenceTimeline` with an explicit zone across BOTH the 23 h
+ * spring-forward and the 25 h fall-back day.
  *
  * Each test uses `vi.useFakeTimers()` to pin `now` so the rolling
  * window is deterministic. `classifyIntakeTiming` tests below the
@@ -343,38 +353,6 @@ describe("calculateCompliance — cadence-aware adapter", () => {
     expect(result.missed).toBe(0);
   });
 
-  it("DST boundary (Europe/Berlin spring-forward) does not inflate the expected count", () => {
-    // Spring-forward in Berlin: 2025-03-30 02:00 → 03:00. A daily
-    // schedule across this boundary must still emit one slot per
-    // local day — not two on the short day and zero on the next.
-    // Pin `now` to Apr-02 noon UTC and run a 7-day window so the
-    // boundary day is inside it. Generate one event per day
-    // (working backwards from NOW − 1 day so each event lands inside
-    // the window) and assert the totalExpected lands at 7 (not 8).
-    vi.setSystemTime(new Date("2025-04-02T12:00:00Z"));
-    const schedules: ComplianceSchedule[] = [
-      { windowStart: "10:00", windowEnd: "11:00", daysOfWeek: null },
-    ];
-    // Slots emitted by the timeline: Mar-27..Apr-02 (Mar-26's window
-    // sits below the rolling-window start of Mar-26 12:00). Generate
-    // one taken event per slot — today's 10:30 slot is also in the
-    // past relative to NOW = 12:00 so it counts as taken too.
-    const events = [];
-    for (let d = 0; d <= 6; d++) {
-      const at = new Date("2025-04-02T10:30:00Z");
-      at.setUTCDate(at.getUTCDate() - d);
-      events.push(eventAt(at, true));
-    }
-    const result = calculateCompliance(events, schedules, 7);
-    // The hard contract: totalExpected stays at 7 — not 8 (which
-    // would mean the DST short day double-emitted) and not 6 (which
-    // would mean the short day silently dropped a slot).
-    expect(result.totalExpected).toBe(7);
-    expect(result.taken).toBe(7);
-    expect(result.missed).toBe(0);
-    expect(result.rate).toBe(100);
-  });
-
   it("filters events outside the rolling window", () => {
     const schedules: ComplianceSchedule[] = [
       { windowStart: "08:00", windowEnd: "09:00", daysOfWeek: null },
diff --git a/vitest.config.mts b/vitest.config.mts
index 6972701c9..caee54df7 100644
--- a/vitest.config.mts
+++ b/vitest.config.mts
@@ -9,6 +9,15 @@ export default defineConfig({
   test: {
     globals: true,
     environment: "node",
+    // Pin the host timezone. CI runs UTC; without this a contributor in any
+    // other zone sees a different suite than the gate does — verified: a
+    // UTC+14 host fails five tests on an otherwise green tree. Pinning makes
+    // local and CI agree, and makes "which zone does this test mean?" an
+    // explicit choice the fixture has to state rather than a property of the
+    // machine it runs on. Tests that need a different HOST zone set
+    // `process.env.TZ` themselves and restore it afterwards (see
+    // `next-due-day-label.test.ts`).
+    env: { TZ: "UTC" },
     // Primes the i18n locale cache so provider mounts resolve every
     // locale synchronously (production gets the active bundle as an RSC
     // prop instead — see src/lib/i18n/load-locale.ts).
diff --git a/vitest.integration.config.mts b/vitest.integration.config.mts
index 4ae52780a..3a36a7989 100644
--- a/vitest.integration.config.mts
+++ b/vitest.integration.config.mts
@@ -13,6 +13,9 @@ export default defineConfig({
   test: {
     globals: true,
     environment: "node",
+    // Same host-timezone pin as the unit config — CI runs UTC, so the
+    // integration contracts must be read against the same clock.
+    env: { TZ: "UTC" },
     include: ["tests/integration/**/*.test.ts"],
     // Container boot + migration apply is slow; give each test a generous
     // budget and beforeAll/afterAll twice that.