[Ietf-calsify] FREQ=MONTHLY when DTSTART = 29/30/31st.

[Mike Samuel]

My objection to your interpretation of
RRULE:FREQ=MONTHLY;BYMONTHDAY=15,30 is that it does not have the
property that

  for every RRULE r (with no count) and a DTSTART d1, if I pick a date
d2 from the series (r, d1)
  then the series (r, d2) should be a tail of (r, d1)


On 19/02/07, Nigel Swinson <Nigel.Swinson at rockliffe.com> wrote:
>
>
> Please could you confirm to me that the following does not include the 15th
> of February:
>
> DTSTART;VALUE=DATE:20070130
> RRULE:FREQ=MONTHLY;BYMONTHDAY=15,30
>
> Nor does this include any date in February:
>
>
>
> DTSTART;VALUE=DATE:20070130
> RRULE:FREQ=MONTHLY;BYMONTHDAY=1,-1
>
>
> Whereas these would:
>
>
> DTSTART;VALUE=DATE:20070115
> RRULE:FREQ=MONTHLY;BYMONTHDAY=15,30
> EXDATE:20070115
>
>
> DTSTART;VALUE=DATE:20070130
> RRULE:FREQ=DAILY;BYMONTHDAY=15,30
>
>
> I don't find the above particularly obvious, so in case it helps, my
> thinking is that you use DTSTART and FREQ to create the initial recurrence
> set, and then refine/expand it using the BYXXX rules.  As there is no 30th
> of Feb, the initial recurrence set contains no dates in February, and hence
> we have no date in february to expand or refine to create additional dates
> in february in the context of a monthly rule.
>
> If I am right, and you also consider the above not particularly obvious,
> perhaps it worth refining this paragraph in RFC2445 to spell out the
> algorithm.  I suggest we move the "invalid date" paragraph down one, and
> then:
>
> http://tools.ietf.org/html/draft-ietf-calsify-rfc2445bis-05#section-3.3.10
>
>       Information, not contained in the rule, necessary to determine the
>       various recurrence instance start time and dates are derived from
>       the Start Time ("DTSTART") component attribute.  For example,
>       "FREQ=YEARLY;BYMONTH=1" doesn't specify a specific day within the
>       month or a time.  This information would be the same as what is
>       specified for "DTSTART".
>
>       Recurrence rules may generate recurrence instances with invalid
>       date (e.g., February 30) or nonexistent local time (e.g., 1:30 AM
>       on a day where the local time is moved forward by an hour at 1:00
>       AM).  Such recurrence instances MUST be ignored and MUST NOT be
>       counted as part of the recurrence set.
> +    Should the combination of DTSTART and FREQ itself create
> +    recurrences that lie on non existant dates, then you end up skipping
> +    entire intervals.  For example "DTSTART;VALUE=DATE:20070130" with
> +    "FREQ=MONTHLY" will never contain any instances in February regardless
> +    of what BYXXX rule parts are specified.
>
>       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.
>
> Cheers
>
>
> Nigel
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