iCalrecurrence

iCalrecurrence is a concept used in calendaring to describe the handling of recurring events according to the iCalendar standard. It encompasses parsing recurrence specifications, expanding them into individual event instances, and applying exceptions and overrides.

In iCalendar, recurrence is expressed primarily with the RRULE property, which uses the FREQ (daily, weekly, monthly, yearly), INTERVAL, COUNT, and UNTIL components, and may be refined with BY rules. Additional dates can be added with RDATE and modified with EXDATE. The DTSTART value sets the start of the first occurrence and may include time zone information via TZID.

A typical iCalrecurrence processor takes a recurrence rule and start date and yields or collects all valid instances within a requested time window, suitable for display in calendars, reminders, or scheduling interfaces. Implementations exist across programming languages and calendar tools, and are used to generate ICS data or to render recurring events stored in databases.

Key design considerations include efficiently handling long or infinite sequences, correctly applying time zone transitions and daylight saving time, resolving overrides to exceptions, and combining multiple recurrence rules.

See also iCalendar, RRULE, RFC 5545, ICS.