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Timezones (was: limits of the generic)

• To: jeni@j...
• Subject: Timezones (was: limits of the generic)
• From: John Cowan <jcowan@r...>
• Date: Sun, 29 Sep 2002 14:08:15 -0400 (EDT)
• Cc: xml-dev@l..., dareo@m... (Dare Obasanjo), jonathan.robie@d... (Jonathan Robie)
• In-reply-to: <9242042658.20020929112007@j...> from "Jeni Tennison" at Sep 29, 2002 11:20:07 AM

Jeni Tennison scripsit:

> Of course when you schedule something down to the particular day, then
> you can *know* what timezone is relevant. So I can tell my friend "my
> plane touches down at 2002-11-29T09:45:00-05:00
> (2002-11-29T14:45:00Z)" (I'll assume we're both geeks, so she
> understands ISO 8601-formatted date/times).

Actually, you can't *know* it.  Washington or Albany might decide to
change the timezone rules between now and then, in which case the UTC
time would change.

Historically, the safest thing is to label the zone "America/New_York",
which reflects a certain historical pattern of timezone use.  This is
not the same as "America/Montreal", because that has a different
pattern of zone switches, and furthermore is under the control of a
different national authority that could cause it to migrate away from
America/New_York, as was the case in 1974-75.

Only past or present events can safely be labeled by a mere offset from
UTC time.  Future events, unless they are *scheduled* in UTC time
(like astronomical events), need more information.  The entirely
non-standard and unofficial (but generally used on all non-Windows
platforms) ADO timezone system recognizes about 400 distinct zones
worldwide, labeled by continent and largest city.

-- 
Not to perambulate              || John Cowan <jcowan@r...>
    the corridors               || http://www.reutershealth.com
during the hours of repose      || http://www.ccil.org/~cowan
    in the boots of ascension.  \\ Sign in Austrian ski-resort hotel

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