[pickers] Fix DST issue with America/Asuncion timezone and AdapterMoment (@flaviendelangle) (#15653)

Co-authored-by: Flavien DELANGLE <[email protected]>

Author: github-actions[bot]

packages/x-date-pickers/src/AdapterMoment/AdapterMoment.ts

@@ -522,14 +522,25 @@ export class AdapterMoment implements MuiPickersAdapter<Moment, string> {
 
     let count = 0;
     let current = start;
+    let currentDayOfYear = current.get('dayOfYear');
     const nestedWeeks: Moment[][] = [];
 
     while (current.isBefore(end)) {
       const weekNumber = Math.floor(count / 7);
       nestedWeeks[weekNumber] = nestedWeeks[weekNumber] || [];
       nestedWeeks[weekNumber].push(current);
 
+      const prevDayOfYear = currentDayOfYear;
       current = this.addDays(current, 1);
+      currentDayOfYear = current.get('dayOfYear');
+
+      // If there is a TZ change at midnight, adding 1 day may only increase the date by 23 hours to 11pm
+      // To fix, bump the date into the next day (add 12 hours) and then revert to the start of the day
+      // See https://github.com/moment/moment/issues/4743#issuecomment-811306874 for context.
+      if (prevDayOfYear === currentDayOfYear) {
+        current = current.add(12, 'h').startOf('day');
+      }
+
       count += 1;
     }
 

packages/x-date-pickers/src/DateCalendar/tests/timezone.DateCalendar.test.tsx

@@ -72,6 +72,12 @@ describe('<DateCalendar /> - Timezone', () => {
       ).to.equal(30);
     });
 
+    // See https://github.com/mui/mui-x/issues/14730
+    it('should not render duplicate days when leaving DST in America/Asuncion', () => {
+      render(<DateCalendar timezone="America/Asuncion" value={adapter.date('2024-10-10')} />);
+      expect(screen.getAllByRole('gridcell', { name: '5' })).to.have.length(1);
+    });
+
     TIMEZONE_TO_TEST.forEach((timezone) => {
       describe(`Timezone: ${timezone}`, () => {
         it('should use timezone prop for onChange when no value is provided', () => {