Events are stored with a naive (UTC) dtstart, but standard .ics exporters
(Google, Apple, Outlook, Fastmail) write the recurrence bound as an absolute
UTC value, e.g. FREQ=DAILY;UNTIL=20240105T090000Z. dateutil refuses to mix a
tz-aware UNTIL with a naive DTSTART ("RRULE UNTIL values must be specified in
UTC when DTSTART is timezone-aware"), so _expand_rrule's except branch swallowed
the ValueError and silently downgraded the event to non-recurring — every
occurrence after the first vanished from the calendar.
When dtstart is naive, strip the trailing Z from UNTIL so it matches the naive
DTSTART before parsing. No effect on tz-aware dtstarts or naive-UNTIL rules.
Adds tests/test_calendar_rrule_until_utc.py — a daily series bounded by a UTC
UNTIL expands to all 5 occurrences (fails before: returns 1, non-recurring).
Co-authored-by: NubsCarson <nubs@nubs.site>
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