How to get iCal to recognize recurring events in iCalendar (ics) files

Thu, Apr 26, 2012

Here is the solution to a problem I recently debugged which I had a very hard time finding any information on via Google. Hopefully this helps someone else in the future.

When programatically generating iCalendar (.ics) files, iCal on OS X (at least on my Snow Leopard version, 4.0.4) seems to refuse to recognize that the event is recurring if the VEVENT contains a RECURRENCE-ID element.

For an event that should repeat on the first Thursday of the month, iCal would only add the first event if the ICS file looked like this:

BEGIN:VEVENT
DTSTAMP:20120425T210028Z
DTSTART;TZID=America/New_York:20120503T180000
DTEND;TZID=America/New_York:20120503T190000
SUMMARY:This event should repeat on first Thursday of the month
RRULE:FREQ=MONTHLY;INTERVAL=1;COUNT=4;BYDAY=1TH
RECURRENCE-ID;TZID=America/New_York:20120503T180000

However, remove the RECURRENCE-ID element and iCal recognizes that this is a recurring event just fine - it adds events on the first Thursday of the month for four months:

BEGIN:VEVENT
DTSTAMP:20120425T210028Z
DTSTART;TZID=America/New_York:20120503T180000
DTEND;TZID=America/New_York:20120503T190000
SUMMARY:This event should repeat on first Thursday of the month
RRULE:FREQ=MONTHLY;INTERVAL=1;COUNT=4;BYDAY=1TH

Outlook 2011 and Google Calendar recognize the first example as hoped for (shows an event that repeats on first Thursday of the month four times). It’s not clear to me from the RFC which behavior is correct - the meaning of RECURRENCE-ID seems confusing.