fix(rrule): prevent DATE-only events from dropping when UNTIL contains a UTC timestamp with positive offset (e.g. Outlook exports)

lib/ical-parser-utils.js

@@ -417,6 +417,25 @@ function normalizeRruleUntil(rruleOnly, startDate, tzUtil) {
   const untilEnd = untilStart + untilMatch[0].length;
 
   if (startDate.dateOnly) {
+    if (timePart && zSuffix) {
+      // When UNTIL is in UTC (e.g. from Outlook: UNTIL=20261019T230000Z for DTSTART=20261020),
+      // the UTC timestamp reflects midnight in the creator's positive-offset timezone.
+      const year = Number(datePart.slice(0, 4));
+      const monthIndex = Number(datePart.slice(4, 6)) - 1;
+      const day = Number(datePart.slice(6, 8));
+      const hours = Number(timePart.slice(1, 3));
+      const minutes = Number(timePart.slice(3, 5));
+      const seconds = Number(timePart.slice(5, 7));
+      const utcTimestamp = Date.UTC(year, monthIndex, day, hours, minutes, seconds);
+      const dtstartTime = Date.UTC(startDate.getFullYear(), startDate.getMonth(), startDate.getDate());
+
+      // If UTC UNTIL is within 24h before dtstart due to timezone crossing midnight:
+      const normalizedDate = (utcTimestamp < dtstartTime && (dtstartTime - utcTimestamp) <= 86_400_000)
+        ? buildDateOnlyStamp(startDate)
+        : datePart;
+      return rruleOnly.slice(0, untilStart) + `UNTIL=${normalizedDate}` + rruleOnly.slice(untilEnd);
+    }
+
     if (timePart) {
       return rruleOnly.slice(0, untilStart) + `UNTIL=${datePart}` + rruleOnly.slice(untilEnd);
     }

test/date-only-rrule-until.test.js

@@ -200,4 +200,41 @@ END:VCALENDAR`;
     const recurrences = event.rrule.all();
     assert.ok(recurrences.length > 0, 'Should have recurrences');
   });
+
+  it('should parse Outlook yearly date-only recurrence with UTC UNTIL offset', function () {
+    const ics = `BEGIN:VCALENDAR
+VERSION:2.0
+PRODID:-//Microsoft Corporation//Outlook 16.0 MIMEDIR//EN
+BEGIN:VEVENT
+SUMMARY:Annual Company Anniversary
+DTSTART;VALUE=DATE:20261020
+DTEND;VALUE=DATE:20261021
+RRULE:FREQ=YEARLY;UNTIL=20261019T230000Z;INTERVAL=1;BYMONTHDAY=20;BYMONTH=10
+UID:test-outlook-rrule-until
+END:VEVENT
+END:VCALENDAR`;
+
+    const parsed = ical.sync.parseICS(ics);
+    const event = Object.values(parsed).find(event_ => event_.summary === 'Annual Company Anniversary');
+    assert.ok(event, 'Event should exist');
+    assert.strictEqual(event.start.dateOnly, true, 'Start should be date-only');
+
+    const instances = ical.expandRecurringEvent(event, {
+      from: new Date('2026-01-01'),
+      to: new Date('2027-01-01'),
+    });
+
+    assert.strictEqual(instances.length, 1, 'Should expand to 1 occurrence');
+    assert.strictEqual(instances[0].start.getFullYear(), 2026);
+    assert.strictEqual(instances[0].start.getMonth(), 9);
+    assert.strictEqual(instances[0].start.getDate(), 20);
+
+    const recurrences = event.rrule.all();
+    assert.strictEqual(recurrences.length, 1, 'Should have 1 recurrence via rrule.all()');
+    const firstDate = new Date(recurrences[0]);
+    assert.strictEqual(firstDate.getFullYear(), 2026);
+    assert.strictEqual(firstDate.getMonth(), 9);
+    assert.strictEqual(firstDate.getDate(), 20);
+  });
 });
+