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iCalendar Recurrence Problems and Recommendations
6. Best Practices
6.3.Clause6.3Reschedules occur in two different varieties (iTIP), rescheduling where the RECURRENCE-ID is supposed to be changed and when RECURRENCE-ID is not supposed to change.
6.3.1.Clause6.3.1Rescheduling of one or more instances where RECURRENCE-ID is not subject to change. In this case, recurrence properties are not sent, so the receiver is expected to keep the current recurrence set and simply reschedule what’s already on the calendar to different dates/times.

6.3.1.2.

Many instance example: Chair sends out a 5 day recurring meeting that repeats Monday through Friday. Chair later reschedules Wednesday and Thursday to a different time. The Wednesday and Thursday instances should retain their original RECURRENCE-ID; this property should not be updated.