Calendar Talk:Recurrence and Exceptions

RECURRENCE-ID: RFC 2445 4.8.4.4 says

  The date/time value is set to the time when the original recurrence
  instance would occur; meaning that if the intent is to change a
  Friday meeting to Thursday, the date/time is still set to the
  original Friday meeting.

This could be useful if there is a matching exception date in the original occurence. To revert the changed time to the normal time for a particular instance, need to identify which EXDATE to remove.

RDATE: note that a separate instance may not be required in the ics file if the only thing has changed is the date. Can use an EXDATE property to note the exception to the recurrence rule and an RDATE to note the new date. (RDATES can also be used for recurrences not based on gregorian calendar, such as those based on lunar or astronomical cycles.)