Questions regarding recurrence rules

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Hi Guys,
I have few questions regarding details of recurrence rules of rfc 2445. (So=
rry=20
if I may sound more clueless than I really am. I have my assumptions, but I=
=20
don't want to influence any answers by bringing too much of my own argument=
s=20
and thoughts).


1) About the BYDAY rule part: Let's start with some examples
  RRULE:FREQ=3DYEARLY;BYDAY=3D3SU
Okay, that's the third sunday of the year. But what exactly is
  RRULE:FREQ=3DYEARLY;BYDAY=3D5SU;BYMONTH=3D2
Is this the fith sunday in the year, if it is also in february? Or is it th=
e=20
fifth sunday in february? And it can be even less clear from rfc 2445. What=
=20
is
  RRULE:FREQ=3DYEARLY;BYDAY=3D5SU;BYMONTH=3D2,7
Is this the fifth sunday of the year, if it's in either February or July?=20
Or is it the fifth sunday of both february and July?
Or is it the fifth sunday of febrary and july together (i.e. in feb if the =
feb=20
in a leap year has 5 sundays, or the first sunday of july in all other=20
years)?

If you think it's the latter, think about the following rule:
  RRULE:FREQ=3DYEARLY;BYWEEKNO=3D4;BYDAY=3D5SU
Is it the fifth sunday of all sundays that are in week #4? Or is it the fif=
th=20
sunday of the year if it is also in week #4?


Also, what does this rule mean:
  RRULE:FREQ=3DYEARLY;BYMONTH=3D3;BYDAY=3D2SU;BYYEARDAY=3D72
I suppose it doesn't mean the 72th day of the year, if that's the 2nd sunda=
y=20
in March. Is there any way to express that recurrence in an RRULE?


Also notice that rfc 2445 says "For example, within
   a MONTHLY rule, +1MO (or simply 1MO) represents the first Monday
   within the month, whereas -1MO represents the last Monday of the
   month. ". So what does=20
  RRULE:FREQ=3DMONTHLY;BYYEARDAY=3D57,64;BYDAY=3D-1MO
mean? The last monday of each month if it's also day #57 or #64 of the year=
=20
(which the quote implies), or the last monday each month in the set of all=
=20
days #57 and #64 of the year. In particular, in 2007, would the recurrence=
=20
set be Feb 27 (=3D#57 of the year, and the last monday in february) without=
 any=20
occurence in march (march 5 is day #64, but it's not the last monday in the=
=20
month, since rfc 2445 says -1MO means last monday of the month)? Or would i=
t=20
be Feb 27 (day #57 of the year, and last monday of all dates that match the=
=20
BYYEARDAY in the february interval) and March 4 ( day #64 of the year and=20
last monday of all dates in March that match the BYYEARDAY).



2) In Section 4.3.10 RFC 2445 says "The COUNT rule part defines the number =
of=20
occurrences at which to range-bound the recurrence. The "DTSTART" property=
=20
value, if specified, counts as the first occurrence.". So look at this rrul=
e:
DTSTART;TZID=3Dwhatever:20050530T120000
RRULE:FREQ=3DWEEKLY;COUNT=3D3;BYMONTH=3D6
The resulting recurrence dates would be
  1) 20050530
  2) 20050630
  3) 20060630

Now if the DTSTART is not specified the quote from above would imply that t=
he=20
start doesn't count as a recurrence, so looking at
DTEND;TZID=3Dwhatever:20050530T150000
DURATION:PT3H
RRULE:FREQ=3DWEEKLY;COUNT=3D3;BYMONTH=3D6
would give the following resulting recurrence dates?
  1) 20050530 (DTSTART is always the first occurence, )
  2) 20050630
  3) 20060630
  4) 20070630 (DTSTART doesn't count towards COUNT)



3) About the DTSTART and recurrence rules: Is the DTSTART always taken to=20
match the rule (even if it doesn't fulfil the BY* parts)? In particular thi=
s=20
is important for EXRULES: Is the DTSTART always the first occurence of the=
=20
EXRULE? If that's the case, the DTSTART will always be excluded as soon as =
at=20
least one EXRULE is present.=20
And since exceptions overrule inclusions, there's no way to have an occuren=
ce=20
on the DTSTART in that case... Is this really the intended behaviour?

Thanks a lot already for your clarifications / discussions.

Cheers,
Reinhold


=2D-=20
=2D-----------------------------------------------------------------
Reinhold Kainhofer, Vienna University of Technology, Austria
email: [email protected], http://reinhold.kainhofer.com/
 * Financial and Actuarial Mathematics, TU Wien, http://www.fam.tuwien.ac.a=
t/
 * K Desktop Environment, http://www.kde.org, KOrganizer / KPilot maintainer

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