feat(settings): add opt-in calendar localization

src/utils/dateTime.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'vitest';
+import {
+  DEFAULT_CALENDAR_PREFERENCE,
+  formatDateTime,
+  isCalendarPreference
+} from './dateTime';
+
+describe('date/time formatting', () => {
+  const instant = new Date('2026-03-21T14:30:00.000Z');
+
+  it('uses Gregorian explicitly by default, including in localized UI languages', () => {
+    const options: Intl.DateTimeFormatOptions = { dateStyle: 'medium', timeStyle: 'short' };
+    expect(formatDateTime(instant, { locale: 'fa', options })).toBe(
+      new Intl.DateTimeFormat('fa-u-ca-gregory', options).format(instant)
+    );
+    expect(DEFAULT_CALENDAR_PREFERENCE).toBe('gregorian');
+  });
+
+  it('supports the opt-in Persian and Hebrew calendars', () => {
+    const options: Intl.DateTimeFormatOptions = { dateStyle: 'long' };
+    expect(formatDateTime(instant, { locale: 'fa', calendar: 'persian', options })).toBe(
+      new Intl.DateTimeFormat('fa-u-ca-persian', options).format(instant)
+    );
+    expect(formatDateTime(instant, { locale: 'he', calendar: 'hebrew', options })).toBe(
+      new Intl.DateTimeFormat('he-u-ca-hebrew', options).format(instant)
+    );
+  });
+
+  it('returns a safe placeholder for malformed timestamps and rejects unknown preferences', () => {
+    expect(formatDateTime('not-a-timestamp', { locale: 'en' })).toBe('-');
+    expect(isCalendarPreference('gregorian')).toBe(true);
+    expect(isCalendarPreference('lunar')).toBe(false);
+    expect(isCalendarPreference(null)).toBe(false);
+  });
+});