diff --git a/apps/calendar-macos/CompassData/Sources/CompassData/NativeCalendarRootModel.swift b/apps/calendar-macos/CompassData/Sources/CompassData/NativeCalendarRootModel.swift
index 57fdb7d3e..4d2bbf688 100644
--- a/apps/calendar-macos/CompassData/Sources/CompassData/NativeCalendarRootModel.swift
+++ b/apps/calendar-macos/CompassData/Sources/CompassData/NativeCalendarRootModel.swift
@@ -1190,3 +1190,15 @@ public final class NativeCalendarRootModel {
         }
     }
 }
+
+extension NativeCalendarRootModel {
+    /// Tests drive calendar-hint sync without mocking the remote list API.
+    internal func applyCalendarsForPresentationTesting(_ updated: [CompassCalendar]) {
+        calendars = updated
+        rebuildPresentation()
+    }
+
+    internal var testingVisibleRangeRefreshIsScheduled: Bool {
+        refreshTask != nil
+    }
+}
diff --git a/apps/calendar-macos/CompassData/Tests/CompassDataTests/NativeCalendarRootModelCalendarHintSyncTests.swift b/apps/calendar-macos/CompassData/Tests/CompassDataTests/NativeCalendarRootModelCalendarHintSyncTests.swift
new file mode 100644
index 000000000..615e2c4a1
--- /dev/null
+++ b/apps/calendar-macos/CompassData/Tests/CompassDataTests/NativeCalendarRootModelCalendarHintSyncTests.swift
@@ -0,0 +1,91 @@
+import CompassData
+import CompassKit
+import XCTest
+@testable import CompassData
+
+@MainActor
+final class NativeCalendarRootModelCalendarHintSyncTests: XCTestCase {
+    func testCalendarHintChangeReanchorsWeekAndSchedulesRangeRefresh() async throws {
+        let priorDefault = NSTimeZone.default
+        defer { NSTimeZone.default = priorDefault }
+        NSTimeZone.default = try XCTUnwrap(TimeZone(identifier: "UTC"))
+
+        let referenceNow = try XCTUnwrap(
+            CompassDateParsing.parseGridDateTime(
+                "2026-11-12T00:23:00.000",
+                timeZoneIdentifier: "UTC"
+            )
+        )
+        let environment = try NativeCalendarEnvironment(inMemoryDatabase: true)
+        let model = NativeCalendarRootModel(environment: environment, now: { referenceNow })
+        model.viewStore.setCalendarTimeZoneHint("America/New_York")
+
+        let nycOnly = [
+            compassCalendar(id: "nyc", name: "NYC", timeZone: "America/New_York", isPrimary: true, isVisible: true),
+        ]
+        model.applyCalendarsForPresentationTesting(nycOnly)
+        let anchorInNewYork = model.viewStore.anchorDate
+        XCTAssertEqual(EffectiveTimeZone.identifier, "America/New_York")
+
+        let hiddenDenverPrimary = [
+            compassCalendar(
+                id: "denver-primary", name: "Primary", timeZone: "America/Denver", isPrimary: true, isVisible: false
+            ),
+            compassCalendar(id: "nyc", name: "NYC", timeZone: "America/New_York", isPrimary: false, isVisible: true),
+        ]
+        model.applyCalendarsForPresentationTesting(hiddenDenverPrimary)
+
+        XCTAssertEqual(EffectiveTimeZone.identifier, "America/Denver")
+        XCTAssertNotEqual(model.viewStore.anchorDate, anchorInNewYork)
+        let expected = CalendarWindowMath.sundayWeekViewStart(containing: referenceNow)
+        XCTAssertEqual(model.viewStore.anchorDate, expected)
+        XCTAssertTrue(model.testingVisibleRangeRefreshIsScheduled)
+    }
+
+    func testHiddenPrimaryDenverWinsOverVisibleNewYorkOnFullList() async throws {
+        let environment = try NativeCalendarEnvironment(inMemoryDatabase: true)
+        let model = NativeCalendarRootModel(environment: environment)
+        model.applyCalendarsForPresentationTesting([
+            compassCalendar(
+                id: "denver-primary", name: "Primary", timeZone: "America/Denver", isPrimary: true, isVisible: false
+            ),
+            compassCalendar(id: "nyc", name: "NYC", timeZone: "America/New_York", isPrimary: false, isVisible: true),
+        ])
+        XCTAssertEqual(EffectiveTimeZone.identifier, "America/Denver")
+    }
+
+    private func compassCalendar(
+        id: String,
+        name: String,
+        timeZone: String,
+        isPrimary: Bool,
+        isVisible: Bool
+    ) -> CompassCalendar {
+        let capabilities = CompassCalendarCapabilities(
+            canInviteAttendees: false,
+            canManage: false,
+            canReadAvailability: true,
+            canReadDetails: true,
+            canWatchEvents: false,
+            canWrite: true,
+            conferenceKinds: []
+        )
+        return CompassCalendar(
+            access: .owner,
+            accountEmail: "a@example.com",
+            backgroundColor: "#ffffff",
+            capabilities: capabilities,
+            conference: nil,
+            createsGoogleMeet: false,
+            description: "",
+            foregroundColor: "#000000",
+            id: id,
+            isActive: true,
+            isPrimary: isPrimary,
+            isVisible: isVisible,
+            name: name,
+            provider: "google",
+            timeZone: IANATimeZone(rawValue: timeZone)
+        )
+    }
+}
diff --git a/apps/calendar-web/src/calendars/calendar.query.test.ts b/apps/calendar-web/src/calendars/calendar.query.test.ts
new file mode 100644
index 000000000..dce0d87ad
--- /dev/null
+++ b/apps/calendar-web/src/calendars/calendar.query.test.ts
@@ -0,0 +1,74 @@
+import { type QueryFunctionContext } from "@tanstack/react-query";
+import { TimeZoneSchema } from "@core/types/domain-primitives";
+import { createMockCalendar } from "@web/__tests__/utils/factories/calendar.factory";
+import { CalendarApi } from "@web/api/calendar.api";
+import { queryClient } from "@web/api/query-client";
+import {
+  calendarQueryKeys,
+  calendarsQueryOptions,
+} from "@web/calendars/calendar.query";
+import * as calendarTimeZoneHint from "@web/timezone/calendar-timezone-hint";
+import {
+  resetEffectiveTimeZoneStoreForTests,
+  setBrowserTimeZoneForTests,
+} from "@web/timezone/effective-timezone.store";
+import { afterEach, describe, expect, it, spyOn } from "bun:test";
+
+function calendarsQueryContext(): QueryFunctionContext<
+  typeof calendarQueryKeys.all
+> {
+  return {
+    client: queryClient,
+    queryKey: calendarQueryKeys.all,
+    signal: new AbortController().signal,
+    meta: undefined,
+  };
+}
+
+describe("calendarsQueryOptions", () => {
+  afterEach(() => {
+    resetEffectiveTimeZoneStoreForTests();
+  });
+
+  it("syncs the calendar timezone hint when the remote list resolves", async () => {
+    const calendars = [
+      createMockCalendar({
+        name: "Work",
+        timeZone: TimeZoneSchema.parse("America/Denver"),
+        isPrimary: true,
+      }),
+    ];
+    const list = spyOn(CalendarApi, "list").mockResolvedValue(calendars);
+    const sync = spyOn(
+      calendarTimeZoneHint,
+      "syncCalendarTimeZoneHintFromCalendars",
+    );
+
+    await calendarsQueryOptions(true).queryFn!(calendarsQueryContext());
+
+    expect(list).toHaveBeenCalledTimes(1);
+    expect(sync).toHaveBeenCalledTimes(1);
+    expect(sync).toHaveBeenCalledWith(calendars);
+
+    list.mockRestore();
+    sync.mockRestore();
+  });
+
+  it("syncs the calendar timezone hint for the anonymous sentinel path", async () => {
+    setBrowserTimeZoneForTests("UTC");
+    const sync = spyOn(
+      calendarTimeZoneHint,
+      "syncCalendarTimeZoneHintFromCalendars",
+    );
+
+    await calendarsQueryOptions(false).queryFn!(calendarsQueryContext());
+
+    expect(sync).toHaveBeenCalledTimes(1);
+    const synced = sync.mock.calls[0]?.[0];
+    expect(synced).toBeDefined();
+    expect(synced).toHaveLength(1);
+    expect(synced![0]!.isVisible).toBe(true);
+
+    sync.mockRestore();
+  });
+});
diff --git a/apps/calendar-web/src/routers/loaders.test.ts b/apps/calendar-web/src/routers/loaders.test.ts
index a35988d77..b0fadc492 100644
--- a/apps/calendar-web/src/routers/loaders.test.ts
+++ b/apps/calendar-web/src/routers/loaders.test.ts
@@ -161,6 +161,14 @@ describe("loadTodayData", () => {
 
     expect(loadTodayData().dateString).toBe("2026-11-11");
   });
+
+  it("uses the browser timezone for today when the calendars cache is empty", () => {
+    setBrowserTimeZoneForTests("America/Chicago");
+    setSystemTime(new Date("2026-06-01T18:00:00.000Z"));
+    queryClient.removeQueries({ queryKey: calendarQueryKeys.all });
+
+    expect(loadTodayData().dateString).toBe("2026-06-01");
+  });
 });
 
 describe("loadDateParam", () => {