fix(calendar): keep yearly events in DTSTART's month when BYMONTH is missing

defaultmodules/calendar/calendarfetcherutils.js

@@ -222,6 +222,42 @@ const CalendarFetcherUtils = {
 		}
 	},
 
+	/**
+	 * Detects yearly rules that restate DTSTART's day-of-month in BYMONTHDAY but omit BYMONTH,
+	 * as exported by several calendar clients (typically for birthdays), e.g.
+	 *
+	 *   DTSTART;VALUE=DATE:20231002
+	 *   RRULE:FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTHDAY=2
+	 *
+	 * RFC 5545 makes BYMONTHDAY an *expanding* rule part for FREQ=YEARLY, so a conforming
+	 * expander returns the 2nd of every month - the event shows up twelve times a year
+	 * instead of once. The clients that emit this, and the web UIs that render it, all show
+	 * it once a year on DTSTART's date, so we follow the author's evident intent.
+	 *
+	 * Detection reads rrule.origOptions, which carries only the parts the author actually
+	 * wrote (not the defaults node-ical fills in), so a missing BYMONTH is unambiguous. It
+	 * stays deliberately narrow, leaving rules that genuinely expand alone: BYMONTHDAY must
+	 * hold exactly one value equal to DTSTART's day-of-month (merely restating DTSTART), and
+	 * no other BYxxx part may shape the recurrence.
+	 * @param {object} event The recurring event object
+	 * @returns {boolean} True if the rule should be confined to DTSTART's month
+	 */
+	isYearlyRuleMissingByMonth (event) {
+		const options = event.rrule?.origOptions;
+		if (!options || options.freq !== "YEARLY") {
+			return false;
+		}
+
+		const isSingleMonthDay = Array.isArray(options.byMonthDay) && options.byMonthDay.length === 1;
+		// Any other BYxxx part means the rule shapes the recurrence on purpose.
+		const hasShapingPart = Boolean(options.byMonth || options.byDay || options.byYearDay || options.byWeekNo || options.bySetPos);
+		if (!isSingleMonthDay || hasShapingPart) {
+			return false;
+		}
+
+		return options.byMonthDay[0] === event.start.getDate();
+	},
+
 	/**
 	 * Expands a recurring event into individual event instances using node-ical.
 	 * Handles RRULE expansion, EXDATE filtering, RECURRENCE-ID overrides, and ongoing events.
@@ -232,6 +268,9 @@ const CalendarFetcherUtils = {
 	 */
 	expandRecurringEvent (event, pastLocalMoment, futureLocalMoment) {
 		const localTimezone = CalendarFetcherUtils.getLocalTimezone();
+		// Drop the eleven spurious months produced by a yearly rule that is missing BYMONTH.
+		const confineToStartMonth = CalendarFetcherUtils.isYearlyRuleMissingByMonth(event);
+		const startMonth = event.start.getMonth();
 
 		return ical
 			.expandRecurringEvent(event, {
@@ -241,6 +280,7 @@ const CalendarFetcherUtils = {
 				excludeExdates: true,
 				expandOngoing: true
 			})
+			.filter((inst) => !confineToStartMonth || inst.start.getMonth() === startMonth)
 			.map((inst) => {
 				let startMoment, endMoment;
 				if (inst.isFullDay) {

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

@@ -517,4 +517,83 @@ END:VCALENDAR`);
 			expect(filteredEvents[0].location).toBe("Berlin");
 		});
 	});
+
+	describe("yearly events that restate DTSTART's day in BYMONTHDAY but omit BYMONTH", () => {
+		// See GitHub issues #2547 and #3047: several calendar clients export a yearly
+		// event (typically a birthday) as FREQ=YEARLY;BYMONTHDAY=<day of DTSTART> without
+		// a BYMONTH part. RFC 5545 makes BYMONTHDAY an *expanding* rule part for YEARLY,
+		// so a conforming expander returns that day in every month - the event then shows
+		// up twelve times a year instead of once.
+
+		const yearConfig = { ...defaultConfig, maximumNumberOfDays: 365 };
+
+		const buildEvent = function (rrule, dtstart = "20231002", dtend = "20231003") {
+			return ical.parseICS(`BEGIN:VCALENDAR
+BEGIN:VEVENT
+DTSTART;VALUE=DATE:${dtstart}
+DTEND;VALUE=DATE:${dtend}
+RRULE:${rrule}
+DTSTAMP:20230425T111027Z
+UID:yearly-bymonthday@example.com
+SUMMARY:Ted Birthday
+END:VEVENT
+END:VCALENDAR`);
+		};
+
+		const monthDaysOf = (events) => events.map((event) => moment(event.startDate, "x").format("MM-DD"));
+
+		it("should occur only in DTSTART's month when BYMONTH is missing", () => {
+			const data = buildEvent("FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTHDAY=2");
+
+			const monthDays = monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig));
+
+			expect(monthDays.length).toBeGreaterThan(0);
+			expect(monthDays).toEqual(monthDays.map(() => "10-02"));
+		});
+
+		it("should still expand a rule that lists several days of the month", () => {
+			// FREQ=YEARLY;BYMONTHDAY=1,3 legitimately expands across the whole year.
+			const data = buildEvent("FREQ=YEARLY;BYMONTHDAY=1,3");
+
+			const months = new Set(monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig)).map((md) => md.slice(0, 2)));
+
+			expect(months.size).toBe(12);
+		});
+
+		it("should still expand a rule that also constrains the weekday", () => {
+			// "Every Friday the 13th" - BYDAY shapes the recurrence, so it must expand.
+			const data = buildEvent("FREQ=YEARLY;BYMONTHDAY=13;BYDAY=FR");
+
+			const monthDays = monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig));
+
+			expect(monthDays.length).toBeGreaterThan(0);
+			expect(monthDays.every((md) => md.endsWith("-13"))).toBe(true);
+			expect(monthDays.some((md) => !md.startsWith("10"))).toBe(true);
+		});
+
+		it("should still expand when BYMONTHDAY does not match DTSTART's day", () => {
+			// The day was not simply restated from DTSTART, so the rule means something else.
+			const data = buildEvent("FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTHDAY=7");
+
+			const months = new Set(monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig)).map((md) => md.slice(0, 2)));
+
+			expect(months.size).toBe(12);
+		});
+
+		it("should keep a well-formed yearly rule with BYMONTH on its single date", () => {
+			const data = buildEvent("FREQ=YEARLY;WKST=MO;INTERVAL=1;BYMONTHDAY=2;BYMONTH=10");
+
+			const monthDays = monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig));
+
+			expect(monthDays).toEqual(["10-02"]);
+		});
+
+		it("should keep a plain yearly rule on its single date", () => {
+			const data = buildEvent("FREQ=YEARLY");
+
+			const monthDays = monthDaysOf(CalendarFetcherUtils.filterEvents(data, yearConfig));
+
+			expect(monthDays).toEqual(["10-02"]);
+		});
+	});
 });