FW: Question about: TWiki.org Service: Date and Time Gateway

Robert Elz kre at munnari.OZ.AU
Tue Feb 15 04:31:05 UTC 2005

    Date:        Mon, 14 Feb 2005 09:18:10 -0500
    From:        "Olson, Arthur David (NIH/NCI)" <olsona at dc37a.nci.nih.gov>
    Message-ID:  <75DDD376F2B6B546B722398AC161106C740377 at nihexchange2.nih.gov>

To expand a little on Paul Eggert's response...

  | I am not sure I can explain the +/- GMT question, someone from the Olson tz
  | database team is probably better in that.

There is no absolute right or wrong here, there are two different
times, local time (wherever) and UTC (GMT).   To reference the difference
between local time and UTC we subtract one from the other.   Which one
is subtracted from makes no real difference to anything.   Given there are
two ways to do a thing, and which way it is done makes no real difference
to anything, different people, naturally, choose different ways.

>From 30 years ago (or more), unix picked UTC-local as the way to get the
offset.   If you do it that way, in the Eastern part of the US, during
winter, you get a +5 hour difference.

The International time standards (notational standards) picked the other
way (whether that's from before or after unix picked its way I have no
idea) - they use local-UTC.   The Eastern time zone in the US is then
-5 hours from UTC.

Most things these days use international time notation, or something
loosely based upon it (so the time zone in the Date field of this message
should be +0700 if nothing alters it - I'm currently 7 hours ahead of

But the time zone in the system I'm running is set at -420 (420 minutes
different from UTC, with UTC behind local time).   That's because it is
a unix system, which does things the other way.

The Olson tz database was developed for unix systems, the names it gives
to zones when represented externally, are generally the unix version of
the time - on a unix system to set US EST, you'd set GMT+5, which would
then cause an e-mail program to put -0500 in the e-mail headers!

For any of this to make sense, you simply must know the convention used.

If times are being displayed in some non-OS specific context, I would
certainly not be showing anyone the unix (now posix) specified format,
that should be confined to where it is needed.

Display timezones always in the international format (if you don't have
a suitable time zone name to display, get the numeric value, and display
it yourself, don't use the timezone name from the olson database).


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