[LEAPSECS] Future time

[Tony Finch]

Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> In message <52DC19FF.9499.11A39558 at dan.tobias.name>, "Daniel R. Tobias" writes:

>

> >When I'm making an advance dinner reservation for 7 PM on October 1,

> >2014 in New York City, I expect that [...]

>

> That used to be the "sensible party position", but it breaks down

> in heaps once you start to schedule tele-conferences etc.


In theory it works fine, if it is done properly. The organiser needs to
pick a primary location, where the local time of the appointment remains
fixed. There might be some secondary locations if some participants are
elsewhere; these are useful for automatically displaying the correct time
of the appointment for each person, and for the organiser to spot if their
proposed time happens to be 04:00 for one of the participants. The
secodary locations are also useful when a TZ rule change occurs: the
scheduler can find all the appointments that are affected by the rule
change, that is, appointments that occur in multiple locations where the
TZ rule change affects the locations differently. This kind of automated
assistance is impossible if you schedule in UTC.

If you schedule in a fixed zone (whether fixed UTC offset, or fixed local
time rules, or fixed Olson tz name) there are situations where a rule
change will invalidate your scheduling. However current scheduling
software requires you to schedule in this broken way because the main
calendaring data format (iCalendar) has the wrong data model. So in
practice it is impossible to schedule things in a way that is robust
against TZ changes.

There are other problems with existing calendaring software such as
getting time zone names wrong, e.g. implying the time zone in Lisbon in
the summer is called GMT.

So in practice scheduling in UTC makes sense.

Longer version of this argument: http://fanf.livejournal.com/104586.html

Tony.
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