[Kde-pim] Libical versus system time zones

[David Jarvie]

On Friday 24 November 2006 12:26, Reinhold Kainhofer wrote:
> Am Freitag, 24. November 2006 11:57 schrieb David Jarvie:
> > It must have been too late at night when I wrote this - of course, the
> > reason that the system time zones are output as RDATE values rather than
> > RRULE values is that the tzfile format used to define system time zones,
> > holds a sequence of time transitions rather than storing rules.
>
> I wouldn't like that approach. We had it with our holiday files and forgot
> to update them, so suddently no holidays were shown any more for some
> regions.

The system time zones on my system contain dates up to 2037, which I think 
might just about be adequate to overcome your reservations :-) It's also just 
possible that predicting time changes so far ahead might be a little 
unrealistic anyway.

> > Another
> > approach to generating the VTIMEZONE component would be to attempt to
> > recognise the date sequence as a rule.
>
> For the Olsen TZ database including various links see
> http://www.twinsun.com/tz/tz-link.htm
>
> In particular, there is a converter from the Olsen DB to VTIMEZONE:
> Vzic:
> http://dspace.dial.pipex.com/prod/dialspace/town/pipexdsl/s/asbm26/vzic/

The only time zone databases we have on the system are the system one (which 
AFAICS is derived from the Olsen database) and the libical one. Since the 
system database is already parsed from the Olsen database into binary form, 
all we can do is choose whether or not to convert it to RRULEs. I've now 
partly implemented this at home - it isn't too difficult to do at least a 
basic conversion and shouldn't impose any significant overhead.

The only question remaining, in my view, is whether to truncate the date range 
contained in the VTIMEZONE to those actually used in the calendar. I propose 
that we should do this, since it would cut out a lot of useless and textually 
long-winded data from the calendar, and would also follow the example of the 
libical database which only starts in 1970.

-- 
David Jarvie.
KAlarm author and maintainer.
http://www.astrojar.org.uk/linux/kalarm.html
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