When setting up an event within the Zimbra Web Client that repeats on the nth day-of-week of a month (e.g. the 1st Tuesday of November - U.S. election day), Lightning displays that event as occurring on every day-of-week of that month (continuing the example, every Tuesday in November).

Setting the event up from within Lightning creates a correct display both for Zimbra and for Lightning.

Reading the ICS files for both, the RRULEs are different:

Lightning says: RRULE:FREQ=YEARLY;BYDAY=1TU;BYMONTH=1
Zimbra says: RRULE:FREQ=YEARLY;BYDAY=TU;BYMONTH=1;BYSETPOS=1

Can anyone tell me if one of these is "more acceptable"? I'll submit the issue to the Lightning bug tracker if both are equally valid, but I'd like confirmation.

(I'm using the Lightning nightly builds, 1.0b2pre 04-Jan-2010 at present).