FW: Time Zone Data File Conventions

Robert Elz kre at munnari.OZ.AU
Fri Oct 17 01:40:50 UTC 1997

    Date:        Thu, 16 Oct 1997 10:35:45 -0400
    From:        "Olson, Arthur David" <OLSONA at dc37a.nci.nih.gov>
    Message-ID:  <9710161510.AA18373 at elsie.nci.nih.gov>

To clarify one more time...

  | 1) You use the concepts of "wall clock", "standard", and "GMT" time in
  | specifying 
  | transition times. I'd like to verify just what these mean. 
  | a) Does "wall clock" mean simply take the time as given applying no
  | corrections?

"wall clock" time is the time someone would see looking at a local clock
on the wall in the appropriate area (assuming it is showing the correct
local time).   This is used in North America, where the rule is "when the
clock gets to 02:00 on the appropriate day, adjust it, forwards or backwards
as needed" (well, that's a paraphrase...)

  | b) Does "standard" mean apply DST time shift to given time to find true
  | time?

"standard" time is the time that would prevail if no DST rule was in
effect in a given area.   This kind of rule is used in Aust, and other
places, where summer time (which is specified as being distinct from
standard time) is defined to begin at 02:00 standard time, and end at
02:00 standard time (which is 03:00 on the clock at the time it shifts)

The 's' suffix to the transition time means that the time has been written
in the file as specified in legislation (or whatever), and that there it
is specified in the area's standard time zone offset.  Clearly, it is trivially
possible to convert between wall clock and standard times, the two are
available to allow the most natural representation to be specified (not
because it is actyally necessary for anything).

  | c) Does "GMT" mean apply GMT offset to given time to get true time?

GMT ('u') means that the time written in the zone file has been written in
UTC (GMT) rather than in local time at all.   Whether that means applying
the GMT offset, or not applying it, depends on what you're starting with.


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