Timezone / recurrence scheduling discussion for Kolab

Andrew McMillan andrew at morphoss.com
Fri Nov 12 22:24:29 CET 2010

On Wed, 2010-11-03 at 12:21 +0000, David Jarvie wrote:

> > how to resolve this. An explanation of the issue can be found here:
> >
> >  http://kolab.org/pipermail/kolab-format/2010-October/001004.html
> >

> I'm not familiar with how Kolab stores its data, so I don't know what
> restrictions there are. From my experience, the only way to make this work
> reliably is to store the recurrence time as a local time with associated
> time zone (which is how the meeting time is actually specified by the
> person organising it), as in iCalendar. Then each client can convert the
> time to its own time zone, without any risk of error.

Hi David,

As long as the desired timezone of the event is known it really doesn't
matter whether the base time is stored in UTC or in the local time in
that timezone - it just matters that you know which one it is.

If it's stored as UTC, and you have the timezone, then you can convert
the base time into to the local time before calculating the recurring
series.  This *does* require you to have accurate historical timezone
information, but that's generally easier than having accurate future
ones anyway :-)

If it's stored as a local time then you *still* need to know the
timezone of that local time in order to be able to calculate the offset
from UTC for each future recurrence, and then localise that to someone
else's timezone.

What's missing from the Kolab format is the timezone.

If I were to design such a format from scratch I know that I would
specify all times to be in the local timezone, but the format already
exists, and it specifies that the time is in UTC. Changing that would
have a large effect on existing software, whereas an additional timezone
element should hopefully be able to be ignored by older versions.

					Andrew McMillan

http://andrew.mcmillan.net.nz/                     Porirua, New Zealand
Twitter: _karora                                  Phone: +64(272)DEBIAN
       Don't engineer in a crisis.  -- Vint Cerf speaking on IPv6

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