fix(web): sync calendar timezone hint before day redirect

apps/calendar-web/src/calendars/calendar.query.ts

@@ -16,6 +16,7 @@ import {
   isSentinelCalendarList,
   synthesizeLocalCalendar,
 } from "@web/calendars/local-calendar.sentinel";
+import { syncCalendarTimeZoneHintFromCalendars } from "@web/timezone/calendar-timezone-hint";
 
 export const calendarQueryKeys = {
   all: ["calendars"] as const,
@@ -36,10 +37,14 @@ export function calendarsQueryOptions(authenticated: boolean) {
       // covers the gap before React re-renders after the session store flips,
       // so a still-mounted anonymous observer cannot write the sentinel back.
       if (!authenticated && !readAuthenticated()) {
-        return [synthesizeLocalCalendar(getLocalCalendarSentinelId())];
+        const local = [synthesizeLocalCalendar(getLocalCalendarSentinelId())];
+        syncCalendarTimeZoneHintFromCalendars(local);
+        return local;
       }
 
-      return CalendarApi.list();
+      const calendars = await CalendarApi.list();
+      syncCalendarTimeZoneHintFromCalendars(calendars);
+      return calendars;
     },
     staleTime: 60_000,
   });
@@ -97,6 +102,7 @@ export async function ensureRemoteCalendars(
 
   const calendars = await client.fetchQuery(calendarsQueryOptions(true));
   if (isSentinelCalendarList(calendars)) return undefined;
+  syncCalendarTimeZoneHintFromCalendars(calendars);
   return calendars;
 }
 

apps/calendar-web/src/routers/loaders.test.ts

@@ -5,6 +5,10 @@ import {
   createRouter,
   isRedirect,
 } from "@tanstack/react-router";
+import { TimeZoneSchema } from "@core/types/domain-primitives";
+import { createMockCalendar } from "@web/__tests__/utils/factories/calendar.factory";
+import { queryClient } from "@web/api/query-client";
+import { calendarQueryKeys } from "@web/calendars/calendar.query";
 import { ROOT_ROUTES } from "@web/common/constants/routes";
 import * as prefetchWeekEvents from "@web/events/queries/prefetch-week-events";
 import {
@@ -16,7 +20,18 @@ import {
   redirectToToday,
   validateWeekDateParam,
 } from "@web/routers/loaders";
-import { afterEach, describe, expect, it, spyOn } from "bun:test";
+import {
+  resetEffectiveTimeZoneStoreForTests,
+  setBrowserTimeZoneForTests,
+} from "@web/timezone/effective-timezone.store";
+import {
+  afterEach,
+  describe,
+  expect,
+  it,
+  setSystemTime,
+  spyOn,
+} from "bun:test";
 
 function getRedirect(fn: () => unknown) {
   try {
@@ -126,6 +141,28 @@ describe("router redirects", () => {
   });
 });
 
+describe("loadTodayData", () => {
+  afterEach(() => {
+    resetEffectiveTimeZoneStoreForTests();
+    queryClient.clear();
+    setSystemTime();
+  });
+
+  it("applies a cached calendar hint before picking today at the merge-queue boundary", () => {
+    setBrowserTimeZoneForTests("UTC");
+    setSystemTime(new Date("2026-11-12T00:23:00.000Z"));
+    queryClient.setQueryData(calendarQueryKeys.all, [
+      createMockCalendar({
+        name: "miscellaneous",
+        timeZone: TimeZoneSchema.parse("America/Denver"),
+        isPrimary: true,
+      }),
+    ]);
+
+    expect(loadTodayData().dateString).toBe("2026-11-11");
+  });
+});
+
 describe("loadDateParam", () => {
   it("shapes the loader data for an already-valid dateString param", () => {
     const result = loadDateParam({

apps/calendar-web/src/routers/loaders.ts

@@ -1,6 +1,9 @@
 import { redirect } from "@tanstack/react-router";
+import { type Calendar } from "@core/types/calendar.contracts";
 import { zYearMonthDayString } from "@core/types/type.utils";
 import dayjs, { type Dayjs } from "@core/util/date/dayjs";
+import { queryClient } from "@web/api/query-client";
+import { calendarQueryKeys } from "@web/calendars/calendar.query";
 import {
   DEFAULT_CALENDAR_ROUTE,
   ROOT_ROUTES,
@@ -11,6 +14,7 @@ import {
   prefetchWeekEventsQuery,
   weekDateStringFromPathname,
 } from "@web/events/queries/prefetch-week-events";
+import { syncCalendarTimeZoneHintFromCalendars } from "@web/timezone/calendar-timezone-hint";
 import { getEffectiveTimeZone } from "@web/timezone/effective-timezone.store";
 
 export interface DayLoaderData {
@@ -38,7 +42,15 @@ export async function loadAuthenticated() {
   return { authenticated };
 }
 
+function syncCachedCalendarTimeZoneHintBeforeToday(): void {
+  const calendars = queryClient.getQueryData<Calendar[]>(calendarQueryKeys.all);
+  if (calendars?.length) {
+    syncCalendarTimeZoneHintFromCalendars(calendars);
+  }
+}
+
 export function loadTodayData(): DayLoaderData {
+  syncCachedCalendarTimeZoneHintBeforeToday();
   const dateInView = dayjs().tz(getEffectiveTimeZone());
   const dateFormat = dayjs.DateFormat.YEAR_MONTH_DAY_FORMAT;
 

apps/calendar-web/src/timezone/CalendarTimeZoneHintSync.tsx

@@ -1,16 +1,13 @@
 import { useLayoutEffect } from "react";
 import { useCalendarsQuery } from "@web/calendars/calendar.query";
-import { pickCalendarTimeZoneHint } from "@web/timezone/calendar-timezone-hint";
-import { setCalendarTimeZoneHint } from "@web/timezone/effective-timezone.store";
+import { syncCalendarTimeZoneHintFromCalendars } from "@web/timezone/calendar-timezone-hint";
 
 /** Keeps the Auto effective zone aligned with connected calendar timezones. */
 export function CalendarTimeZoneHintSync() {
   const { data: calendars } = useCalendarsQuery();
 
   useLayoutEffect(() => {
-    setCalendarTimeZoneHint(
-      calendars ? pickCalendarTimeZoneHint(calendars) : null,
-    );
+    syncCalendarTimeZoneHintFromCalendars(calendars);
   }, [calendars]);
 
   return null;

apps/calendar-web/src/timezone/calendar-timezone-hint.test.ts

@@ -1,7 +1,39 @@
 import { TimeZoneSchema } from "@core/types/domain-primitives";
+import dayjs from "@core/util/date/dayjs";
 import { createMockCalendar } from "@web/__tests__/utils/factories/calendar.factory";
-import { pickCalendarTimeZoneHint } from "@web/timezone/calendar-timezone-hint";
-import { describe, expect, it } from "bun:test";
+import {
+  pickCalendarTimeZoneHint,
+  syncCalendarTimeZoneHintFromCalendars,
+} from "@web/timezone/calendar-timezone-hint";
+import {
+  getEffectiveTimeZone,
+  resetEffectiveTimeZoneStoreForTests,
+  setBrowserTimeZoneForTests,
+} from "@web/timezone/effective-timezone.store";
+import { afterEach, describe, expect, it, setSystemTime } from "bun:test";
+
+describe("syncCalendarTimeZoneHintFromCalendars", () => {
+  afterEach(() => {
+    resetEffectiveTimeZoneStoreForTests();
+    setSystemTime();
+  });
+
+  it("applies denver today before navigation when the browser is UTC at the merge-queue boundary", () => {
+    setBrowserTimeZoneForTests("UTC");
+    setSystemTime(new Date("2026-11-12T00:23:00.000Z"));
+    syncCalendarTimeZoneHintFromCalendars([
+      createMockCalendar({
+        name: "miscellaneous",
+        timeZone: TimeZoneSchema.parse("America/Denver"),
+        isPrimary: true,
+      }),
+    ]);
+    expect(getEffectiveTimeZone()).toBe("America/Denver");
+    expect(dayjs().tz(getEffectiveTimeZone()).format("YYYY-MM-DD")).toBe(
+      "2026-11-11",
+    );
+  });
+});
 
 describe("pickCalendarTimeZoneHint", () => {
   it("prefers the primary calendar zone", () => {

apps/calendar-web/src/timezone/calendar-timezone-hint.ts

@@ -1,5 +1,6 @@
 import { type Calendar } from "@core/types/calendar.contracts";
 import { isValidTimeZone } from "@web/timezone/browser-timezone";
+import { setCalendarTimeZoneHint } from "@web/timezone/effective-timezone.store";
 import { isUtcLikeTimeZone } from "@web/timezone/utc-like-timezone";
 
 /**
@@ -32,3 +33,12 @@ export function pickCalendarTimeZoneHint(
 
   return null;
 }
+
+/** Keep Auto effective zone aligned with connected calendars (see CalendarTimeZoneHintSync). */
+export function syncCalendarTimeZoneHintFromCalendars(
+  calendars: readonly Calendar[] | null | undefined,
+): void {
+  setCalendarTimeZoneHint(
+    calendars ? pickCalendarTimeZoneHint(calendars) : null,
+  );
+}

apps/calendar-web/src/views/Day/components/Calendar/dayCalendarColumns.util.test.ts

@@ -216,6 +216,68 @@ describe("calendarIdsWithVisibleEventsOnDay", () => {
     resetEffectiveTimeZoneStoreForTests();
   });
 
+  it("shows every stress calendar column when viewed day matches denver event day", () => {
+    setBrowserTimeZoneForTests("UTC");
+    const stressCalendars = Array.from({ length: 12 }, (_, index) =>
+      makeCalendar({
+        id: `f${String(index).padStart(23, "0")}` as Calendar["id"],
+        name: `stress-${index}`,
+        isPrimary: index === 0,
+      }),
+    );
+    const dateInViewDenver = dayjs.tz("2026-11-11T17:23:00", "America/Denver");
+    const timed = stressCalendars.map((calendar, index) =>
+      gridEvent({
+        _id: String(50 + index),
+        calendarId: calendar.id,
+        startDate: "2026-11-11 09:00:00.000",
+        endDate: "2026-11-11 10:00:00.000",
+        timeZone: "America/Denver",
+      }),
+    );
+
+    expect(
+      getDayViewCalendars({
+        allDayEvents: [],
+        calendars: stressCalendars,
+        dateInView: dateInViewDenver,
+        hiddenEventIds: new Set(),
+        timedEvents: timed,
+      }),
+    ).toHaveLength(12);
+  });
+
+  it("falls back to primary when viewed utc day mismatches denver stress events", () => {
+    setBrowserTimeZoneForTests("UTC");
+    const stressCalendars = Array.from({ length: 12 }, (_, index) =>
+      makeCalendar({
+        id: `f${String(index).padStart(23, "0")}` as Calendar["id"],
+        name: `stress-${index}`,
+        isPrimary: index === 0,
+      }),
+    );
+    const dateInViewUtc = dayjs.tz("2026-11-12T00:23:00", "UTC");
+    const timed = stressCalendars.map((calendar, index) =>
+      gridEvent({
+        _id: String(50 + index),
+        calendarId: calendar.id,
+        startDate: "2026-11-11 09:00:00.000",
+        endDate: "2026-11-11 10:00:00.000",
+        timeZone: "America/Denver",
+      }),
+    );
+
+    expect(
+      getDayViewCalendars({
+        allDayEvents: [],
+        calendars: stressCalendars,
+        dateInView: dateInViewUtc,
+        hiddenEventIds: new Set(),
+        timedEvents: timed,
+      }),
+    ).toHaveLength(1);
+  });
+
   it("keeps both day columns at the merge-queue UTC boundary (browser UTC, no calendar hint)", () => {
     setBrowserTimeZoneForTests("UTC");
     const primary = makeCalendar({