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

[Peter Thoeny]

Dear Robert and Paul,

thank you for the clarification.

TWiki.org's gateway already states "The names are extensions to POSIX 
defined by the Olson tz database" and "Greenwich Mean Time: 
(GMT/UCT/UTC/Universal/Zulu; positive numbers count hours west of GMT)"

Which is enough explanation; to avoid confusion I do not add a note 
about difference of International time standards and POSIX.

Best regards,
Peter


Robert Elz wrote:
> 
>     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
> UTC).
> 
> 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).
> 
> kre

--
   * Peter Thoeny                           Peter at Thoeny.com
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