Message to `tz' mailing list

Jesper Norgaard Welen jnorgard at
Fri Feb 17 07:01:51 UTC 2006

This is not ticking in on my computer screen, e.g. I don't understand
the argument that IsGmt and IsStd are irrelevant when you only want to
figure out wall clock time making a program reading the tz file.
However, I may have misunderstood Raghavendra or Arthur David Olson.

In my view the program has to take it into account explicitly, so that
2:00w should correspond to wall clock time, but 2:00u and 2:00s should
be converted to wall clock time base on GMT offset and daylight saving

Let's look at Mexico City. It has GMT offset -6 and daylight saving
offset +1 between First Sunday of April at 2:00 and Last Sunday of
October at 2:00. Both of these could be stated as 2:00w instead if
desired. But if we use u or s it changes the meaning:

Last Sunday of October 2:00u means that it was 21:00w the day before in
Mexico because in October before DST ends there is 5 hour difference to
GMT (-6+1).

Last Sunday of October 1:00s means that it was 2:00w because we have a
daylight saving offset of +1 just before DST ends. But the next second
we will have 1:00:01s and 1:00:01w because now the daylight saving
offset is no longer in effect.

And Last Sunday of October 2:00s would therefore be 2:00w for the second
time that day.

I don't know if this was what you were discussing?

Jesper Nørgaard Welen

-----Original Message-----
From: Olson, Arthur David (NIH/NCI) [E]
[mailto:olsona at] 
Sent: Thursday, February 16, 2006 08:30
To: tz at
Subject: RE: Message to `tz' mailing list

> I am confused about the use of the UTC/local indicators and the
standard/wall indicators in a tzfile.  According to tzfile(5), they "are
used when
> a time zone file is used in handling POSIX-style time zone environment
variables".  Does this mean that these indicators are irrelevant when
> local time using only a tzfile (without looking at the environment).

That's correct: the ttisgmt and ttisstd indicators can be ignored if
you're simply trying to figure out what wall clock time is for the zone
that's associated with the file.

(The indicators are there for the case when the time zone file is used
as a "template" for the DST rules to be used when handling POSIX-style
TZ environment variables; you need to know whether the transition time
is defined in terms of UTC, local wall clock time, or local standard
time to be able to know when translating the transition time for use in
another zone.)


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