Recurrence Rule API
Expand and describe RFC 5545 recurrence rules — the RRULE that powers calendar repeats. The expand endpoint takes an RRULE and a start date-time and returns the next occurrence dates, correctly handling FREQ (daily, weekly, monthly, yearly and the finer hourly/minutely/secondly), INTERVAL (every 2 weeks…), COUNT and UNTIL, BYDAY including ordinals like 2MO or -1FR (so "the last Friday of the month" or "the third Sunday of June"), BYMONTHDAY including negatives (-1 for the last day of the month), BYMONTH and WKST. The describe endpoint turns a rule into a plain-English sentence such as "every week on Monday, Wednesday and Friday, 10 times". Everything is computed locally in UTC and deterministically, so it is instant, private and identical on every machine. Ideal for scheduling and booking systems, calendar and reminder apps, billing and subscription cycles, job and report scheduling, and showing customers when something next happens. Pure local computation — no key, no third-party service, instant. Live, nothing stored. 3 endpoints. This expands the recurrence rule; to build a downloadable .ics calendar event use an iCalendar API, and for plain date arithmetic use a date-time API.
api.oanor.com/rrule-api
