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Question: Week Number Semantics


RFC 2445 references the concept of "Week Number" in the "BYWEEKNO"
recurrence grammar. In addition, ISO 8601 references the concept of "Week
Number". It looks like the views of ISO 8601 are a subset of those in RFC
2445. Does everyone agree with the implications of the extended view taken
by RFC 2445 as it relates to how a CUA should display "Week Number" in a
"Weekly View" UI?

RFC 2445 indicates that:

   The BYWEEKNO rule part specifies a COMMA character (US-ASCII decimal
   44) separated list of ordinals specifying weeks of the year. Valid
   values are 1 to 53 or -53 to -1. This corresponds to weeks according
   to week numbering as defined in [ISO 8601]. A week is defined as a
   seven day period, starting on the day of the week defined to be the
   week start (see WKST). Week number one of the calendar year is the
   first week which contains at least four (4) days in that calendar
   year. This rule part is only valid for YEARLY rules. For example, 3
   represents the third week of the year.

ISO 8601 indicates that:

   3.17  week, calendar: A seven day period within a calendar year,
   starting on a Monday and identified by its ordinal number within
   the year; the first calendar week of the year is the one that
   includes the first Thursday of that year. In the Gregorian
   calendar, this is equivalent to the week which includes 4 January.

Taken together, it appears that ISO 8601 assumes that for the purpose of
calculating the "week number", the week _always_ starts on a Monday. On the
other hand, RFC 2445 clearly allows for a week to start on any day of the
week. Thus, a RFC 2445 "week number" might vary by "start of the week".

The problem that I see is that a CUA's UI may wish to display "Week Number"
in a "Week View" of the calendar. Is "Week Number" a canonical calendar
concept that _always_ must assume Monday as the start of the week? For
example, as is often used across multinational manufacturing corporations or
equivalent "teaming" relationships; where a corporation's factory calendar
is computed, based on weeks of the year. Or is this a calendar concept that
is sensitive to the CUA's local definition of "start of the week" and can
yield different presentations?

It would appear that if CUAs from different vendors are to be iCalendar
"standards" based they should behave the same. Is this right?

If so, then should we view "Week Number" as a non-permutable concept? Or
should the display of "Week Number" vary as the "start of the week" varies?

So end-user feedback on this point would be helpful.

-- Frank Dawson