Re: [calsify] SKIP was Re: AD review of draft-ietf-calext-rscale-03

[Marten Gajda <[e-mail]>]

@Ken: Sorry for the double post, I hit the wrong reply button

Am 03.02.2015 um 19:13 schrieb Ken Murchison:
> On 02/03/2015 12:58 PM, Ken Murchison wrote:
>> On 02/03/2015 12:51 PM, Cyrus Daboo wrote:
>>> Hi Ken,
>>>
>>> --On February 3, 2015 at 12:38:52 PM -0500 Ken Murchison 
>>> <[e-mail]> wrote:
>>>
>>>> I believe we can get by with just a single SKIP and your previous 
>>>> rules
>>>> to handle both cases above (using Hebrew leap month):
>>>>
>>>> RRULE:FREQ=YEARLY;RSCALE=HEBREW;SKIP=FORWARD;BYMONTH=5L;BYMONTHDAY=30
>>>>
>>>> Would yield 1 Nisan (skipping forward both a month and day)
>>>>
>>>>
>>>> RRULE:FREQ=YEARLY;RSCALE=HEBREW;SKIP=FORWARD;BYMONTH=5L;BYMONTHDAY=-1
>>>>
>>>> Would yield 29 Adar (skipping forward just a month)
>>>>
>>>
>>> Actually I am not convinced about that since the spec says that SKIP 
>>> is applied AFTER all other rule parts are processed (except for 
>>> BYSETPOS, COUNT and UNTIL). So the BYMONTHDAY=-1 refers to the last 
>>> day of the invalid month, 30 Adar I, which then becomes the invalid 
>>> date 30 Adar after the month skip and then 1 Nisan after the day skip.
>>
>> Hmm, I think you're right based on the current wording.  I don't 
>> recall exactly how we decided on that wording, but perhaps it might 
>> needs to be adjusted.
>
> Looking at RFC 5545:
>
>       If multiple BYxxx rule parts are specified, then after evaluating
>       the specified FREQ and INTERVAL rule parts, the BYxxx rule parts
>       are applied to the current set of evaluated occurrences in the
>       following order: BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY, BYDAY,
>       BYHOUR, BYMINUTE, BYSECOND and BYSETPOS; then COUNT and UNTIL are
>       evaluated.
>
>
> I'm wondering if we shouldn't have SKIP applied both immediately 
> following BYMONTH and BYMONTHDAY evaluation.  I don't think we want 
> the other BYxxx rule parts to be operating on invalid dates at any 
> point.  Thoughts?
>
The only reason I can come up with right now, is that a rule with RSCALE 
& SKIP=OMIT could have different results than the same rule without 
RSCALE and SKIP.

example:

DTSTART;VALUE=DATE:20120229
RRULE:FREQ=YEARLY;BYMONTH=2;BYDAY=SU

-> all Sundays in February, every year

DTSTART;VALUE=DATE:20120229
RRULE:FREQ=YEARLY;RSCALE=GREGORIAN;BYMONTH=2;BYDAY=SU;SKIP=OMIT

-> all Sundays in February, but only in leap years, since SKIP would 
filter Feb. 29th in non-leap years before BYDAY is evaluated



Marten

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*Marten Gajda*
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