fix: skip native Date fast path when local timezone is overridden via setLocalTimeZone

packages/@internationalized/date/src/conversion.ts

@@ -17,7 +17,7 @@ import {AnyCalendarDate, AnyDateTime, AnyTime, Calendar, DateFields, Disambiguat
 import {CalendarDate, CalendarDateTime, Time, ZonedDateTime} from './CalendarDate';
 import {constrain} from './manipulation';
 import {getExtendedYear, GregorianCalendar} from './calendars/GregorianCalendar';
-import {getLocalTimeZone, isEqualCalendar} from './queries';
+import {getLocalTimeZone, isEqualCalendar, isLocalTimeZoneOverridden} from './queries';
 import {Mutable} from './utils';
 
 export function epochFromDate(date: AnyDateTime): number {
@@ -42,7 +42,9 @@ export function getTimeZoneOffset(ms: number, timeZone: string): number {
   }
 
   // Fast path: for local timezone after 1970, use native Date.
-  if (ms > 0 && timeZone === getLocalTimeZone()) {
+  // Skip this fast path if the local timezone was explicitly overridden via setLocalTimeZone,
+  // since native Date always uses the browser's timezone, not the overridden one.
+  if (ms > 0 && timeZone === getLocalTimeZone() && !isLocalTimeZoneOverridden()) {
     return new Date(ms).getTimezoneOffset() * -60 * 1000;
   }
 
@@ -124,7 +126,9 @@ export function toAbsolute(date: CalendarDate | CalendarDateTime, timeZone: stri
   }
 
   // Fast path: if the time zone is the local timezone and disambiguation is compatible, use native Date.
-  if (timeZone === getLocalTimeZone() && disambiguation === 'compatible') {
+  // Skip this fast path if the local timezone was explicitly overridden via setLocalTimeZone,
+  // since native Date always uses the browser's timezone, not the overridden one.
+  if (timeZone === getLocalTimeZone() && disambiguation === 'compatible' && !isLocalTimeZoneOverridden()) {
     dateTime = toCalendar(dateTime, new GregorianCalendar());
 
     // Don't use Date constructor here because two-digit years are interpreted in the 20th century.

packages/@internationalized/date/src/index.ts

@@ -66,6 +66,7 @@ export {
   getLocalTimeZone,
   setLocalTimeZone,
   resetLocalTimeZone,
+  isLocalTimeZoneOverridden,
   startOfMonth,
   startOfWeek,
   startOfYear,

packages/@internationalized/date/src/queries.ts

@@ -130,6 +130,7 @@ export function getHoursInDay(a: CalendarDate, timeZone: string): number {
 }
 
 let localTimeZone: string | null = null;
+let localTimeZoneOverride = false;
 
 /** Returns the time zone identifier for the current user. */
 export function getLocalTimeZone(): string {
@@ -142,14 +143,21 @@ export function getLocalTimeZone(): string {
 
 /** Sets the time zone identifier for the current user. */
 export function setLocalTimeZone(timeZone: string): void {
+  localTimeZoneOverride = true;
   localTimeZone = timeZone;
 }
 
 /** Resets the time zone identifier for the current user. */
 export function resetLocalTimeZone(): void {
+  localTimeZoneOverride = false;
   localTimeZone = null;
 }
 
+/** Returns whether the local time zone has been explicitly overridden via `setLocalTimeZone`. */
+export function isLocalTimeZoneOverridden(): boolean {
+  return localTimeZoneOverride;
+}
+
 /** Returns the first date of the month for the given date. */
 export function startOfMonth(date: ZonedDateTime): ZonedDateTime;
 export function startOfMonth(date: CalendarDateTime): CalendarDateTime;

packages/@internationalized/date/tests/conversion.test.js

@@ -10,9 +10,9 @@
  * governing permissions and limitations under the License.
  */
 
-import {BuddhistCalendar, CalendarDate, CalendarDateTime, EthiopicAmeteAlemCalendar, EthiopicCalendar, GregorianCalendar, HebrewCalendar, IndianCalendar, IslamicCivilCalendar, IslamicTabularCalendar, IslamicUmalquraCalendar, JapaneseCalendar, PersianCalendar, TaiwanCalendar, Time, toCalendar, toCalendarDate, toCalendarDateTime, toTime, ZonedDateTime} from '..';
+import {BuddhistCalendar, CalendarDate, CalendarDateTime, EthiopicAmeteAlemCalendar, EthiopicCalendar, GregorianCalendar, HebrewCalendar, IndianCalendar, IslamicCivilCalendar, IslamicTabularCalendar, IslamicUmalquraCalendar, JapaneseCalendar, PersianCalendar, resetLocalTimeZone, setLocalTimeZone, TaiwanCalendar, Time, toCalendar, toCalendarDate, toCalendarDateTime, toTime, ZonedDateTime} from '..';
 import {Custom454Calendar} from './customCalendarImpl';
-import {fromAbsolute, possibleAbsolutes, toAbsolute, toDate} from '../src/conversion';
+import {fromAbsolute, getTimeZoneOffset, possibleAbsolutes, toAbsolute, toDate} from '../src/conversion';
 
 describe('CalendarDate conversion', function () {
   describe('toAbsolute', function () {
@@ -522,4 +522,51 @@ describe('CalendarDate conversion', function () {
       expect(toTime(dateTime)).toEqual(new Time(8, 23, 10, 80));
     });
   });
+
+  describe('setLocalTimeZone', function () {
+    afterEach(() => {
+      resetLocalTimeZone();
+    });
+
+    it('should use the overridden timezone in getTimeZoneOffset instead of native Date', function () {
+      let ms = new Date('2020-06-15T12:00:00Z').getTime();
+
+      // Get the offset using the Intl-based slow path for a non-local timezone
+      let expectedOffset = getTimeZoneOffset(ms, 'Etc/GMT-10');
+
+      // Now override the local timezone to 'Etc/GMT-10' and verify it still computes correctly
+      setLocalTimeZone('Etc/GMT-10');
+      let actualOffset = getTimeZoneOffset(ms, 'Etc/GMT-10');
+
+      expect(actualOffset).toBe(expectedOffset);
+    });
+
+    it('should use the overridden timezone in toAbsolute instead of native Date', function () {
+      let date = new CalendarDateTime(2020, 6, 15, 12, 0, 0);
+
+      // Get the expected result using the Intl-based slow path for a non-local timezone
+      let expected = toAbsolute(date, 'Etc/GMT-10');
+
+      // Now override the local timezone and verify it still computes correctly
+      setLocalTimeZone('Etc/GMT-10');
+      let actual = toAbsolute(date, 'Etc/GMT-10');
+
+      expect(actual).toBe(expected);
+    });
+
+    it('should produce correct results after resetLocalTimeZone', function () {
+      let ms = new Date('2020-06-15T12:00:00Z').getTime();
+      let tz = Intl.DateTimeFormat().resolvedOptions().timeZone;
+
+      // Get the offset before any override
+      let offsetBefore = getTimeZoneOffset(ms, tz);
+
+      setLocalTimeZone('Etc/GMT-10');
+      resetLocalTimeZone();
+
+      // After reset, the fast path should be restored and produce the same result
+      let offsetAfter = getTimeZoneOffset(ms, tz);
+      expect(offsetAfter).toBe(offsetBefore);
+    });
+  });
 });

packages/@internationalized/date/tests/queries.test.js

@@ -25,6 +25,7 @@ import {
   isEqualMonth,
   isEqualYear,
   IslamicUmalquraCalendar,
+  isLocalTimeZoneOverridden,
   isSameDay,
   isSameMonth,
   isSameYear,
@@ -371,4 +372,26 @@ describe('queries', function () {
       expect(getLocalTimeZone()).toBe(systemTimeZone);
     });
   });
+
+  describe('isLocalTimeZoneOverridden', function () {
+    afterEach(() => {
+      resetLocalTimeZone();
+    });
+
+    it('returns false by default', function () {
+      expect(isLocalTimeZoneOverridden()).toBe(false);
+    });
+
+    it('returns true after setLocalTimeZone', function () {
+      setLocalTimeZone('America/Denver');
+      expect(isLocalTimeZoneOverridden()).toBe(true);
+    });
+
+    it('returns false after resetLocalTimeZone', function () {
+      setLocalTimeZone('America/Denver');
+      expect(isLocalTimeZoneOverridden()).toBe(true);
+      resetLocalTimeZone();
+      expect(isLocalTimeZoneOverridden()).toBe(false);
+    });
+  });
 });