[Ietf-calsify] RRULEs, TimeZones

[Mark Swanson]

On October 13, 2004 12:23 pm, Helge Hess wrote:
> On Oct 13, 2004, at 18:01, Mark Swanson wrote:
> >> How do you store a recurrence rule for "every monday 11:00 PST" in
> >> UTC?
> >> Due to DST shifts, 11:00 PST translates to different UTC values
> >> depending on the week (18:00 UTC (UTC-7?) in winter and 19:00 UTC
> >> (UTC-6) in summer).
> >
> > The key is in DTSTART. Since you have a UTC value at an instance of
> > time you
> > can compensate for the changing timezone offsets at any set instance
> > in the
> > future.
>
> Yes, if you know the timezone the event was created in. Or in other
> words, you can save the time values in any timezone, including UTC, but
> you need to specify the timezone required for DST and historic
> calculations.

It involves some work, but there is a way to do it without having to know what 
timezone the event is created in.

> > The key is the fact that the presentation layer can convert 11:00am
> > April 22,
> > 2004 Canadian/Eastern into a UTC value and back again.
>
> Of course, but the presentation layer needs to know that the event was
> created in (and is intended for) "Candian/Eastern" time.

Perhaps "(and is intended for)" is also a key factor. F.E. I assume that 
someone flying to Vancouver for a meeting would ask his presentation layer to 
display his events for that day in the Pacific timezone. I believe this to be 
logical.

> An "each Monday at 11:00 MST" is different to "each Monday at 23:00
> CET" even though the UTC starttime might match (DST rules are
> different, so a German flying to Arizona for a meeting at 11:00 MST
> will arrive at the incorrect time).

Still a presentation issue. Perhaps this will help:

1. TZ=PST, recurring monday meeting: 11:00AM created by Joe
2. TZ=EST, Bob's calendar shows a 2:00PM recurring monday meeting.
3. Bob flies to the PST timezone, and sets his presentation timezone to PST, 
which shows his meeting at 11:00AM.

If that isn't the issue Helge/Reinhold, perhaps it's a deeper issue of how 
some folks are creating instances of the recurrence set from the 
DTSTART/RRULE definition.

Cheers.


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