RRULE debugger

Expand an iCalendar recurrence rule, and find out where implementations disagree about it.

Rule a rule, or paste a whole VEVENT

DTSTART from the paste, or type it

Occurrences

Compare with optional — a second rule, to see exactly what an edit changed

Explain one date optional — why is this date in the list, or missing from it?

Try:

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The expander is a brute-force reading of RFC 5545 §3.3.10 written from the specification rather than ported from an existing library. It is scored against a published cross-implementation corpus on every change (results). The divergence notes come from measured findings, each naming the implementation and version it was observed on.

Source and issues on GitHub. Found a case it gets wrong? That is the most useful thing you can send.