Dates: month names run from January to December in every timezone

West of UTC the list of month names began with December and had no February: it was built from a UTC midnight
on January 1st, which the formatter reads in local time as December 31st, and stepping that 31st by a month
skipped February. The birthday popup saves the month a name stands at, so there "January" saved February, and
the bubbles that name a month by its number named the wrong one. Each name is now taken from the middle of its
month in local time, as the week days next to it already are.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: morethanwords

src/helpers/date.ts

@@ -44,11 +44,11 @@ export function getWeekDays() {
 
 export function getMonths() {
   const dateTimeFormat = I18n.getDateTimeFormat({month: 'long'});
-  const date = new Date(Date.UTC(2017, 0, 1));
+  // The middle of each month in local time, as the formatter reads it: a UTC midnight on
+  // January 1st is still December west of Greenwich, and stepping that 31st skipped February.
   const out: string[] = [];
   for(let i = 0; i < 12; ++i) {
-    out.push(capitalizeFirstLetter(dateTimeFormat.format(date)));
-    date.setMonth(date.getMonth() + 1);
+    out.push(capitalizeFirstLetter(dateTimeFormat.format(new Date(2017, i, 15))));
   }
   return out;
 }

src/tests/getWeekDays.test.ts

@@ -0,0 +1,36 @@
+import {getMonths, getWeekDays} from '@helpers/date';
+
+// A fresh formatter per call, so each one picks up the timezone the test sets.
+vi.mock('@lib/langPack', () => ({
+  default: {
+    getDateTimeFormat: (options: Intl.DateTimeFormatOptions) => new Intl.DateTimeFormat('en', options)
+  }
+}));
+
+const ZONES = ['America/Los_Angeles', 'UTC', 'Asia/Tokyo'];
+
+const tz = process.env.TZ;
+afterEach(() => {
+  process.env.TZ = tz;
+});
+
+describe('getWeekDays', () => {
+  // The schedule it labels (OpeningHours.getDaysHours) starts on Monday whatever
+  // the timezone, so the labels have to as well.
+  test.each(ZONES)('starts on Monday in %s', (zone) => {
+    process.env.TZ = zone;
+    expect(getWeekDays()).toEqual(['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']);
+  });
+});
+
+describe('getMonths', () => {
+  // The birthday popup saves the month a name stands at here (index + 1), and the
+  // chat names a month by its number from the same list.
+  test.each(ZONES)('runs from January to December in %s', (zone) => {
+    process.env.TZ = zone;
+    expect(getMonths()).toEqual([
+      'January', 'February', 'March', 'April', 'May', 'June',
+      'July', 'August', 'September', 'October', 'November', 'December'
+    ]);
+  });
+});