Subject: Recurrence ambiguities

From: Damon Chaplin <[email-masked]>
Date: Sat, 15 Jul 2000 03:52:15 +0100

While implementing recurrence expansion I noticed a few problems:

o The BYDAY modifier is ambiguous. In a YEARLY frequency with BYMONTH and/or
  BYWEEKNO set, does the BYDAY modifier apply to the year day, the month day,
  or the week day? I'd guess it applies to the month day or week day.

o Should BYWEEKNO and BYMONTH be mutually exclusive? They make no sense
  together. (This would also avoid part of the previous problem.)

o Is it possible to use BYMONTHDAY in a WEEKLY frequency, or BYYEARDAY in
  a MONTHLY or WEEKLY frequency? I suppose it is possible, but it doesn't
  really make any sense. Maybe the spec should say that they aren't
  applicable to those frequencies (as it says that BYWEEKNO is only
  applicable to YEARLY, (even though it would also make sense in DAILY)).

o BYSETPOS is limited to 366 values (or maybe 999 values if the comment doesn't
  count as part of the spec!), yet a complete expansion for every second in
  every day of the year would result in much larger sets. (Though I realize
  it is unlikely that this limit is reached.) If it is an arbitrary limit
  then maybe the spec should say so.

o The algorithm to calculate the first week in the year may be awkward,
  since some people/countries may be used to other numbering schemes.
  Maybe there should be a choice of a few schemes for this. (Though I'm
  not sure how many people/countries this affects.)

Damon