[Ietf-calsify] RRULEs, TimeZones

[Reinhold Kainhofer]

Mark Swanson wrote:
> On October 13, 2004 12:14 pm, Reinhold Kainhofer wrote:
> > Ahm, for two persons in two different time zones this will not work.
> 
> It does. We'll get to it in a couple of emails I'm sure.

To be honest, I'd really love to see this work. Because then all our problems 
would have vanished... But so far, I haven't seen 

> > Again use the example of my last mail: every monday at 01:30 in Timezone
> > Europe/Vienna. If the start date is in summer time, the event is on sunday
> > 23:30 UTC. No imagine somebody in the US views this event. The event start
> > day will be on a sunday, so for the US person the event will only be
> 
> If your RRULE specifies weekly every Monday, the US person will see the
> Monday  1 second after his DTSTART, so he will see the correct Monday.

Yes, that's what he'll see, which is wrong. See below.

> I'd like to pick a different example because I read this as: you say the
> event is on Monday, then you say the US person viewing the event on a 
> Monday is the wrong day?

Yes, he'll see the event on monday. However, the event really occurs on Monday 
01:30 CEST, which is about *Sunday afternoon* in the US. Oops. 
I really said the event is on monday - for a person in CEST. But monday in 
CEST (UTC+0200) is not necessarily monday for a person in the US!

RRULEs are highly dependent on the local time zone. If you specify a 
recurrence on monday, this will no longer be true in general for a different 
time zone.

And translating RRULES from local time to UTC is not possible either. E.g. 
00:30 localtime on the first monday each month will be either 23:30 UTC of 
the first sunday, or of the last sunday of the previous month. I don't see 
any way to represent this as a RRULE in UTC.

So, to be correct, all recurrence calculation need to be done in the original 
time zone of the event, and only then can the results be translated to the 
time zone of the presentation layer.

Reinhold
-- 
------------------------------------------------------------------
Reinhold Kainhofer, Vienna, Austria
email: reinhold at kainhofer.com, http://reinhold.kainhofer.com/
 * Financial and Actuarial Mathematics, TU Wien, http://www.fam.tuwien.ac.at
 * K Desktop Environment, http://www.kde.org/, KOrganizer / KPilot maintainer
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