Parse an external ICS feed and correctly expand recurring events, applying RRULE, EXDATE, and RECURRENCE-ID overrides
domain: datatracker.ietf.org · 5 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
Parse the feed with an RFC 5545-aware icalendar library rather than hand-rolled line splitting, since folded lines, escaped text, and multi-value properties are easy to get wrong manually.
For each VEVENT with an RRULE, expand occurrences using the combined rule set: RRULE plus any RDATE-added dates, minus any EXDATE-excluded dates.
Match any separate VEVENT that shares the same UID but carries a RECURRENCE-ID to the corresponding generated occurrence, and use that override's fields (time, summary, etc.) in place of the computed occurrence.
Resolve each event's effective timezone from its DTSTART/DTEND TZID parameter against the feed's VTIMEZONE block (or treat as UTC if a trailing Z is present, or floating if neither is present) before displaying or storing occurrence times.
Cap or paginate the expansion window for RRULEs with no COUNT/UNTIL (effectively infinite recurrence) so parsing doesn't attempt to materialize unbounded future occurrences.
Known gotchas
Skipping the RECURRENCE-ID override step causes moved or edited occurrences to display twice (once via naive RRULE expansion, once as the standalone override) or to be missed entirely.
Some feeds omit a VTIMEZONE block despite using TZID references to non-UTC zones (relying on the consuming client to know the IANA zone by name) — a strict parser that requires an embedded VTIMEZONE will fail on these otherwise-common real-world feeds.
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