[Ietf-calsify] Struggling with BYxxx

Reinhold Kainhofer reinhold at kainhofer.com
Sat Jun 23 15:26:20 PDT 2007

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Am Samstag, 23. Juni 2007 schrieb Mr. Demeanour:
> Hi,
> What is the correct interpretation of a YEARLY rule in which BYMONTH and
> BYYEARDAY are both specified?
> I can't find this described anywhere, but my interpretation would be to
> /expand/ in accordance with the BYMONTH part, and then to /expand/ in
> accordance with the BYYEARDAY part. So consider the rule
> I would expect this rule to expand to two Christmases per year, on the
> 25th of June and December respectively, and one New Year's Day.
> Is that correct?

I don't think so. The dates matched by the rule have to fulfill all the BY* 
restrictions. In your case, there are not dates that are the first day of the 
year and lie in June or december...

Or to make it more formal according to the evaluation order in the RFC:
For year YY, the recurrence set is generated as:
BYMONTH: Only all days in June or December
BYYEARDAY: only those days in the set that are the first day of the year 
(which are none, so our recurrence set is empty here, already).
BYMONTHDAY: No effect, since the set is already empty.

The BY* rule parts must all be matched by a date/time in the recurrence set, 
but the BY* rule part values (e.g. 6,12 for BYMONTH here) are or'ed together.


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Reinhold Kainhofer, Vienna University of Technology, Austria
email: reinhold at kainhofer.com, http://reinhold.kainhofer.com/
 * Financial and Actuarial Mathematics, TU Wien, http://www.fam.tuwien.ac.at/
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