feat(lint): refuse UTC day keys outside the time zone module

`instant.toISOString().slice(0, 10)` and parsing a key back as UTC
midnight are the idioms behind every day-boundary fix this year: the
evening west of UTC and the small hours east of it land on the
neighbouring day. A new rule, healthlog/no-utc-day-key, refuses them in
src/ outside src/lib/tz and tests, and refuses a disable directive for
itself that is not next-line or gives no reason.

The time zone module now names every legitimate form: userDayKey for a
person's day, dateOnlyKey / dateOnlyAtNoonUtc / dayKeyAsUtcMidnight for
date-only values and day labels, daysBetweenDateKeys / weekdayOfDateKey /
isCalendarDateKey / statedDateKey for key arithmetic and parsing.

Of the 133 sites the rule found, 84 are gone. Most moved onto a named
helper with no change in behaviour; these changed behaviour:

- cycle calendar, insights, Coach snapshot and prediction refresh start
  their lookbacks at the first instant of the user's day
- a model-transcribed date like "June 10, 2026" no longer lands a day
  early in a process east of UTC (documents and lab scans)
- a custom report range covers exactly the picked days in the profile
  zone instead of starting the evening before west of UTC
- date-of-birth pickers cap at the user's today, not the UTC one
- an allergy onset date is stored at noon UTC like other date-only values
- the report's cycle summary, the Coach's usual BP range and the intake
  projector's rollup recompute use the user's day
- vaccination dates render as dates instead of the previous evening

49 remain, each with its reason on the line above: 39 UTC by design (15
file name stamps, rollup and object keys, the UTC-day score engines) and
10 marked baseline. A guard test requires a classified reason on every
directive and holds the baseline to a ceiling that only goes down.

src/components/cycle/bbt-chart.tsx

@@ -44,6 +44,7 @@ import { prefersReducedMotion } from "@/lib/charts/reduced-motion";
 import { cn } from "@/lib/utils";
 import type { CalendarDay, CervicalMucus, OvulationTest } from "./types";
 import { PHASE_HUE, OVULATION_HUE } from "./phase-tokens";
+import { shiftDateKey } from "@/lib/tz/format";
 
 /** Trailing fallback window (days) when no MENSTRUAL-anchored cycle is active. */
 const FALLBACK_WINDOW_DAYS = 35;
@@ -97,9 +98,7 @@ export function BbtChart({
   // recent temperatures.
   const fromDate = useMemo(() => {
     if (cycleStartDate) return cycleStartDate;
-    const fallback = new Date(ymdToMs(today));
-    fallback.setUTCDate(fallback.getUTCDate() - FALLBACK_WINDOW_DAYS);
-    return fallback.toISOString().slice(0, 10);
+    return shiftDateKey(today, -FALLBACK_WINDOW_DAYS);
   }, [cycleStartDate, today]);
 
   const points = useMemo<BbtPoint[]>(() => {

src/components/links/vault-document-options.ts

@@ -31,6 +31,7 @@ import {
   DOCUMENT_LIST_MAX_LIMIT,
   type InboundDocumentDto,
 } from "@/lib/validations/inbound-documents";
+import { daysBetweenDateKeys } from "@/lib/tz/format";
 
 /**
  * How many pages the picker walks. At the route's ceiling of 100 that is a
@@ -38,8 +39,6 @@ import {
  */
 const VAULT_PICKER_MAX_PAGES = 10;
 
-const DAY_MS = 24 * 60 * 60 * 1000;
-
 /** The fields the option list reads off a document. */
 export type VaultDocument = Pick<
   InboundDocumentDto,
@@ -82,10 +81,12 @@ export function isNearAnchor(
   anchorIso: string | null,
 ): boolean {
   if (!documentDay || !anchorIso) return false;
-  const day = Date.parse(`${documentDay.slice(0, 10)}T00:00:00.000Z`);
-  const anchor = Date.parse(`${anchorIso.slice(0, 10)}T00:00:00.000Z`);
-  if (Number.isNaN(day) || Number.isNaN(anchor)) return false;
-  return Math.abs(day - anchor) <= ENCOUNTER_SUGGEST_WINDOW_DAYS * DAY_MS;
+  const gap = daysBetweenDateKeys(
+    anchorIso.slice(0, 10),
+    documentDay.slice(0, 10),
+  );
+  if (Number.isNaN(gap)) return false;
+  return Math.abs(gap) <= ENCOUNTER_SUGGEST_WINDOW_DAYS;
 }
 
 /**

src/components/measurement-reminders/vorsorge-section.tsx

@@ -98,6 +98,11 @@ import {
   useMeasurementReminderMutations,
   type MeasurementReminder,
 } from "@/hooks/use-measurement-reminders";
+import {
+  detectBrowserTimezone,
+  shiftDateKey,
+  userDayKey,
+} from "@/lib/tz/format";
 
 // v1.18.1 (V3) — the active-measurement set, grouped so the dropdown
 // doesn't read as a flat 15-item list. Mirrors the Zod allow-list in
@@ -177,8 +182,7 @@ function toDateInputValue(iso: string | null): string {
   if (!iso) return "";
   const d = new Date(iso);
   if (Number.isNaN(d.getTime())) return "";
-  const offset = d.getTimezoneOffset();
-  return new Date(d.getTime() - offset * 60 * 1000).toISOString().slice(0, 10);
+  return userDayKey(d, detectBrowserTimezone());
 }
 
 /**
@@ -839,12 +843,11 @@ export function postponeOffsetTargets(
   // in between moves it an hour, which flips the date for anyone acting near
   // midnight. Anchoring today's local date at UTC noon and stepping whole
   // UTC days keeps every offset on the day its label names.
-  const anchor = new Date(`${isoDayInTz(nowMs, tz)}T12:00:00.000Z`);
-  return SNOOZE_CHIP_DAYS.map((days) => {
-    const target = new Date(anchor);
-    target.setUTCDate(target.getUTCDate() + days);
-    return { days, date: target.toISOString().slice(0, 10) };
-  });
+  const today = isoDayInTz(nowMs, tz);
+  return SNOOZE_CHIP_DAYS.map((days) => ({
+    days,
+    date: shiftDateKey(today, days),
+  }));
 }
 
 function intervalProgress(

src/components/medications/sections/data-portability-row.tsx

@@ -66,6 +66,7 @@ export function DataPortabilityRow({
         intake: includeIntake ? "true" : "false",
         medicationId,
       }).toString();
+      // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
       const stamp = new Date().toISOString().slice(0, 10);
       await downloadFromUrl(
         `/api/export/medications?${query}`,

src/components/mood/use-linked-day-context.ts

@@ -30,6 +30,7 @@ import { useQuery } from "@tanstack/react-query";
 import { apiGet } from "@/lib/api/api-fetch";
 import { queryKeys } from "@/lib/query-keys";
 import type { LinkedDayContext } from "@/lib/mood/linked-context";
+import { userDayKey } from "@/lib/tz/format";
 
 /**
  * The browser's own zone — the right answer for an entry that does not exist
@@ -42,9 +43,7 @@ export function browserTimezone(): string {
 /** `YYYY-MM-DD` for a `datetime-local` value, or today when it is empty. */
 export function dayOfLocalInput(value: string): string {
   if (value.length >= 10) return value.slice(0, 10);
-  const now = new Date();
-  const local = new Date(now.getTime() - now.getTimezoneOffset() * 60 * 1000);
-  return local.toISOString().slice(0, 10);
+  return userDayKey(new Date(), browserTimezone());
 }
 
 /**

src/components/onboarding/baseline-fields.tsx

@@ -16,6 +16,7 @@ import type {
   HeightDraft,
   HeightUnitAdapter,
 } from "@/lib/profile/height-unit-display";
+import { detectBrowserTimezone, userDayKey } from "@/lib/tz/format";
 
 /**
  * The four profile inputs of the onboarding baseline step.
@@ -184,7 +185,7 @@ export function BaselineFields({
           id={IDS.dateOfBirth}
           value={value.dateOfBirth}
           onChange={(next) => onChange("dateOfBirth", next)}
-          max={new Date().toISOString().slice(0, 10)}
+          max={userDayKey(new Date(), detectBrowserTimezone())}
           autoComplete="bday"
           aria-invalid={errors.dateOfBirth ? true : undefined}
           aria-describedby={

src/components/records/allergy-form.tsx

@@ -27,6 +27,7 @@ import {
 import { useTranslations } from "@/lib/i18n/context";
 
 import type { AllergyDTO } from "@/lib/records/dto";
+import { dateOnlyAtNoonUtc, isCalendarDateKey } from "@/lib/tz/date-only";
 
 const TEXT_MAX_LENGTH = 2000;
 /** Select sentinel for the optional "not assessed" severity. */
@@ -41,11 +42,14 @@ interface AllergyFormProps {
   footerSlot?: HTMLElement | null;
 }
 
-/** ISO date input (YYYY-MM-DD) → start-of-day UTC instant, or undefined. */
+/**
+ * ISO date input (YYYY-MM-DD) → the instant a date-only value is stored at
+ * (noon UTC), or undefined. UTC midnight read back as the previous day west
+ * of UTC.
+ */
 function toInstant(value: string): string | undefined {
-  if (!value) return undefined;
-  const d = new Date(`${value}T00:00:00.000Z`);
-  return Number.isNaN(d.getTime()) ? undefined : d.toISOString();
+  if (!isCalendarDateKey(value)) return undefined;
+  return dateOnlyAtNoonUtc(value).toISOString();
 }
 
 /**

src/components/settings/account-section/index.tsx

@@ -33,7 +33,11 @@ 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, DEFAULT_TIMEZONE } from "@/lib/tz/format";
+import {
+  detectBrowserTimezone,
+  DEFAULT_TIMEZONE,
+  userDayKey,
+} from "@/lib/tz/format";
 import { apiFetchRaw, apiGet } from "@/lib/api/api-fetch";
 import {
   recentProofErrorMessage,
@@ -53,6 +57,7 @@ import {
 } from "./account-section-utils";
 import { AvatarSection } from "./avatar-section";
 import { SetupCard } from "./setup-card";
+import { dateOnlyKey } from "@/lib/tz/date-only";
 
 export { resolveInitialTimezone } from "./account-section-utils";
 
@@ -145,7 +150,7 @@ export function AccountSection() {
       email: user.email ?? "",
       height: heightAdapter.toDraft(user.heightCm),
       dateOfBirth: user.dateOfBirth
-        ? new Date(user.dateOfBirth).toISOString().slice(0, 10)
+        ? dateOnlyKey(new Date(user.dateOfBirth))
         : "",
       gender: user.gender ?? "",
       timezone: resolveInitialTimezone(user.timezone, detectBrowserTimezone()),
@@ -528,7 +533,7 @@ export function AccountSection() {
                   setDateOfBirth(next);
                   clearFieldError("dateOfBirth");
                 }}
-                max={new Date().toISOString().slice(0, 10)}
+                max={userDayKey(new Date(), detectBrowserTimezone())}
                 aria-invalid={fieldErrors.dateOfBirth ? true : undefined}
                 aria-describedby={
                   fieldErrors.dateOfBirth ? "dob-error" : undefined

src/components/settings/environment-section.tsx

@@ -227,7 +227,10 @@ export function EnvironmentSection() {
   // user can still type an earlier start, but pre-home days resolve to SKIP on
   // the server — to fill the deep past they add an explicit location period.
   const effectiveBackfillStart =
-    backfillStart || (home?.since ? userDayKey(new Date(home.since), tz) : "");
+    // `since` stays on the server's day for the home (a UTC day key in the
+    // environment module), so the default start matches what the worker
+    // compares against.
+    backfillStart || home?.since?.slice(0, 10) || "";
 
   return (
     <div className="space-y-6">

src/components/settings/export-section.tsx

@@ -246,6 +246,7 @@ function CsvCard({
         since: since || undefined,
         until: until || undefined,
       });
+      // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
       const stamp = new Date().toISOString().slice(0, 10);
       const filename = `${filenamePrefix}-${stamp}.csv`;
       await downloadFromUrl(`${endpoint}${query}`, filename);
@@ -329,6 +330,7 @@ function MedicationsCsvCard() {
         until: until || undefined,
         intake: includeIntake ? "true" : "false",
       });
+      // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
       const stamp = new Date().toISOString().slice(0, 10);
       await downloadFromUrl(
         `/api/export/medications${query}`,
@@ -447,6 +449,7 @@ function FullBackupCard() {
   }
 
   async function downloadOnce() {
+    // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
     const stamp = new Date().toISOString().slice(0, 10);
     if (encrypt) {
       const res = await throwIfReproofRequired(

src/components/settings/report-selection/report-visit-offer.tsx

@@ -25,6 +25,7 @@ import { Button } from "@/components/ui/button";
 import { useEncounters } from "@/hooks/use-encounters";
 import { useFormatters, useTranslations } from "@/lib/i18n/context";
 import type { EncounterKind } from "@/generated/prisma/client";
+import { detectBrowserTimezone, userDayKey } from "@/lib/tz/format";
 
 /**
  * How far ahead a booked visit is treated as the reason for this report.
@@ -36,9 +37,7 @@ export const REPORT_VISIT_HORIZON_DAYS = 14;
 
 /** The local `yyyy-MM-dd` a `DateField` takes, in the reader's own day. */
 function localDay(iso: string): string {
-  const at = new Date(iso);
-  const offset = at.getTimezoneOffset();
-  return new Date(at.getTime() - offset * 60_000).toISOString().slice(0, 10);
+  return userDayKey(new Date(iso), detectBrowserTimezone());
 }
 
 export function ReportVisitOffer({

src/components/settings/report-selection/selection-panel.tsx

@@ -66,6 +66,13 @@ import { orderLeaves } from "@/lib/report-selection/selection";
 
 import { ReportScopePicker } from "./report-scope-picker";
 import { ScopeSummary } from "./scope-summary";
+import { startOfLocalDayKey } from "@/lib/tz/local-day";
+import {
+  DEFAULT_TIMEZONE,
+  detectBrowserTimezone,
+  shiftDateKey,
+  validTimezoneOr,
+} from "@/lib/tz/format";
 
 type ExportFormat = "pdf" | "fhir" | "package";
 
@@ -78,6 +85,12 @@ class ReportRequestError extends Error {}
 export function HealthRecordExportPanel() {
   const { t, locale } = useTranslations();
   const { user } = useAuth();
+  // The report reads its window in the profile zone (the server's
+  // `reportTz`), so the picked days are cut there too.
+  const reportTz = validTimezoneOr(
+    user?.timezone ?? detectBrowserTimezone(),
+    DEFAULT_TIMEZONE,
+  );
   const queryClient = useQueryClient();
 
   const [format, setFormat] = useState<ExportFormat>(
@@ -155,8 +168,15 @@ export function HealthRecordExportPanel() {
     const range =
       customRange && startDate && endDate
         ? {
-            startDate: new Date(`${startDate}T00:00:00Z`).toISOString(),
-            endDate: new Date(`${endDate}T23:59:59Z`).toISOString(),
+            // The picked days are the user's days: from the first instant
+            // of the first to the last instant of the last, in their zone.
+            // UTC midnight started the window the previous evening west of
+            // UTC, and the report read it as one day more.
+            startDate: startOfLocalDayKey(startDate, reportTz).toISOString(),
+            endDate: new Date(
+              startOfLocalDayKey(shiftDateKey(endDate, 1), reportTz).getTime() -
+                1,
+            ).toISOString(),
           }
         : { days };
     const res = await throwIfReproofRequired(
@@ -190,6 +210,7 @@ export function HealthRecordExportPanel() {
     const url = URL.createObjectURL(blob);
     const a = document.createElement("a");
     a.href = url;
+    // eslint-disable-next-line healthlog/no-utc-day-key -- UTC by design: file name stamp, not a day shown or compared
     a.download = `healthlog-health-record-${new Date()
       .toISOString()
       .slice(0, 10)}.${ext}`;

src/components/vaccinations/vaccination-document-suggestion.tsx

@@ -60,7 +60,8 @@ export function VaccinationDocumentSuggestion({
     const name = candidate.antigenSlug
       ? t(`vaccinations.catalog.${candidate.antigenSlug}`)
       : (candidate.vaccineName ?? t("vaccinations.suggestion.unnamed"));
-    return `${name} · ${format.date(candidate.occurredAt)}`;
+    // A date stored at UTC midnight: render the date, not the instant.
+    return `${name} · ${format.date(`${candidate.occurredAt.slice(0, 10)}T12:00:00.000Z`)}`;
   };
 
   const onPick = (candidate: VaccinationSuggestion) => {

src/components/vaccinations/vaccination-form.tsx

@@ -35,6 +35,7 @@ import type { Practitioner } from "@/hooks/use-practitioners";
 import type { Vaccination, VaccinationWriteBody } from "./use-vaccinations";
 import { CatalogPicker } from "./catalog-picker";
 import { VaccinationDocumentPicker } from "./vaccination-document-picker";
+import { dayKeyAsUtcMidnight, isCalendarDateKey } from "@/lib/tz/date-only";
 
 /** The seven anatomical sites, mirroring the Prisma `VaccinationSite` enum. */
 export const VACCINATION_SITES = [
@@ -116,8 +117,10 @@ export function draftFromVaccination(row: Vaccination): VaccinationDraft {
 /** The day, at UTC midnight — an Impfpass carries dates, never times. */
 export function draftInstant(day: string): string | null {
   if (!/^\d{4}-\d{2}-\d{2}$/.test(day)) return null;
-  const parsed = new Date(`${day}T00:00:00.000Z`);
-  return Number.isNaN(parsed.getTime()) ? null : parsed.toISOString();
+  if (!isCalendarDateKey(day)) return null;
+  // Stored at UTC midnight by the column's own convention; every reader
+  // takes the date part.
+  return dayKeyAsUtcMidnight(day).toISOString();
 }
 
 /** Does the draft carry at least one identity arm? */

src/components/vaccinations/vaccination-list.tsx

@@ -198,7 +198,11 @@ function DoseRow({
             ) : null}
           </div>
           <div className="text-muted-foreground flex flex-wrap items-center gap-x-2 gap-y-0.5 text-xs">
-            <span>{format.date(record.occurredAt)}</span>
+            <span>
+              {/* A date, stored at UTC midnight: render its date part, not
+                  the instant (the previous evening west of UTC). */}
+              {format.date(`${record.occurredAt.slice(0, 10)}T12:00:00.000Z`)}
+            </span>
             {record.lotNumber ? (
               <span data-slot="vaccination-lot">
                 {t("vaccinations.row.lot", { lot: record.lotNumber })}