fix(calendar): correct day-of-week for full-day recurring events across DST

Full-day recurring events were showing on wrong day after DST transitions.
rrule.js returns UTC dates that shift calendar days when offset changes.

Fix: Extract UTC date components and interpret as local calendar dates,
preserving the intended day regardless of DST offset changes.

Added test for weekly Monday event crossing DST boundary.

Fixes: #4003
Regression from: c2ec6fc

Author: KristjanESPERANTO

modules/default/calendar/calendarfetcherutils.js

@@ -82,8 +82,12 @@ const CalendarFetcherUtils = {
 		// rrule.js returns UTC dates with tzid cleared, so we interpret them in the event's original timezone
 		return dates.map((date) => {
 			if (isFullDayEvent) {
-				// For all-day events, anchor to calendar day in event's timezone
-				return moment.tz(date, eventTimezone).startOf("day");
+				// For all-day events, extract UTC date components and interpret as local date
+				// This prevents DST transitions from shifting the date to the previous/next day
+				const utcYear = date.getUTCFullYear();
+				const utcMonth = date.getUTCMonth();
+				const utcDate = date.getUTCDate();
+				return moment.tz([utcYear, utcMonth, utcDate], eventTimezone);
 			}
 			// For timed events, preserve the time in the event's original timezone
 			return moment.tz(date, "UTC").tz(eventTimezone, true);

tests/unit/modules/default/calendar/calendar_fetcher_utils_spec.js

@@ -111,5 +111,47 @@ END:VEVENT`);
 			expect(januaryFirst[0].toISOString(true)).toContain("09:00:00.000+01:00");
 			expect(julyFirst[0].toISOString(true)).toContain("09:00:00.000+02:00");
 		});
+
+		it("should return correct day-of-week for full-day recurring events across DST transitions", () => {
+			// Test case for GitHub issue #3976: recurring full-day events showing on wrong day
+			// This happens when DST transitions change the UTC offset between occurrences
+			const data = ical.parseICS(`BEGIN:VCALENDAR
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:20251027
+DTEND;VALUE=DATE:20251028
+RRULE:FREQ=WEEKLY;WKST=SU;COUNT=3
+DTSTAMP:20260103T123138Z
+UID:[email protected]
+SUMMARY:Weekly Monday Event
+END:VEVENT
+END:VCALENDAR`);
+
+			const event = data["[email protected]"];
+
+			// Simulate calendar with timezone (e.g., from X-WR-TIMEZONE or user config)
+			// This is how MagicMirror handles full-day events from calendars with timezones
+			event.start.tz = "America/Chicago";
+
+			const pastMoment = moment("2025-10-01");
+			const futureMoment = moment("2025-11-30");
+
+			// Get moments for the recurring event
+			const moments = CalendarFetcherUtils.getMomentsFromRecurringEvent(event, pastMoment, futureMoment, 0);
+
+			// All occurrences should be on Monday (day() === 1) at midnight
+			// Oct 27, 2025 - Before DST ends
+			// Nov 3, 2025 - After DST ends (this was showing as Sunday before the fix)
+			// Nov 10, 2025 - After DST ends
+			expect(moments).toHaveLength(3);
+			expect(moments[0].day()).toBe(1); // Monday
+			expect(moments[0].format("YYYY-MM-DD")).toBe("2025-10-27");
+			expect(moments[0].hour()).toBe(0); // Midnight
+			expect(moments[1].day()).toBe(1); // Monday (not Sunday!)
+			expect(moments[1].format("YYYY-MM-DD")).toBe("2025-11-03");
+			expect(moments[1].hour()).toBe(0); // Midnight
+			expect(moments[2].day()).toBe(1); // Monday
+			expect(moments[2].format("YYYY-MM-DD")).toBe("2025-11-10");
+			expect(moments[2].hour()).toBe(0); // Midnight
+		});
 	});
 });