Proposed Text for RFC2445 Issue #15: Recurrence Rules

[Eric Busboom <eric@xxxxxxxxxxxxxxxxxx>]

This is a proposal to address RFC2445 Issue number 15, from the issues
list online at:
 
        http://www.softwarestudio.org/iCal/2445Issues.html
 
Issue #15 is:

	Restrictions on RECUR rules
	http://www.imc.org/ietf-calendar/mail-archive/msg03656.html

This proposal will be open for discussion for two weeks, so please send
any comments soon.
 
eric.
 
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[This proposed text is an addition to RFC2445, section 4.3.10. It is
intended to disallow recurrence rules that are nonsensical or for which
there is no common interpretation. The goal is to make it easier to
implement recurrences and make it more likely that differenct
implementations will be interoperable]

[ NOTE: If you want to dispute this proposal *please* include an
illustrative recurrence rule and an accompanying description. This is a
really complex subject and we need to keep the discussions grounded in
actual use and need. ]

There are two types of BYxxx rules; date BYxxx rule parts that specify
time periods equal to or longer than a day, and time BYxxx rule parts that
specify the time periods less than a day. The day BYxxx rule parts are :
BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY and BYDAY. The time BYxxx rule
parts are BYHOUR, BYMINUTE, and BYSECOND. BYSETPOS is neither a date rule
part nor time rule part. 

Not all date BYxxx rule parts can appear simultaneously in a single
recurrence rule. Date BYxxx rule part restrictions are:

	[a] If the BYYEARDAY part appears in a rule , no other date rule
	part may appear.

	[b] BYWEEKNO and BYMONTH rule parts may not both appear. 
	[ except when week covers two months?]

Furthermore, not all date rule parts may be used with all recurrence
intervals.

	[c] For MONTHLY recurrences (FREQ=MONTHLY) neither BYYEARDAY nor
	BYWEEKNO may appear.

	[d] For WEEKLY recurrences (FREQ=WEEKLY) neither BYMONTHDAY nor
	BYYEARDAY may appear.
 
There are no restrictions regarding combinations of date rule parts and
time rule parts. There are no restrictions with using any combination of
time rule parts


[ The BYYEARDAY part, for a given year, effectively specifies the month,
day of month, and week number, so restriction [a] eliminates ambiguity
between the day of year and other specifiers. 

Restrictions [b] is similar, since the week number specifies the month for
a given year. There may be a probably with this restriction when the week
overlaps two months, though.

]