[tz] tzdata isstd question

Ken Murchison murch at andrew.cmu.edu
Fri Oct 30 17:52:40 UTC 2015

On 10/30/2015 01:42 PM, Random832 wrote:
> My message disappeared. I've determined this was because I changed my
> primary email address and wasn't recognized as subscribed to the list,
> but it would have been nice to have received a notification of this as I
> have for other lists.
> On Thu, Oct 29, 2015, at 16:14, Paul Eggert wrote:
>> Sorry, I must be missing something, but I am not following the question.
>> For that absolute transition, there's no difference between 02:00
>> standard time, 03:00 local time, and 08:00 UTC; they're all the same
>> thing, and the "s" in "2:00s" is purely for convenience in the source
>> data.
> The binary data does include information about the difference for the
> purpose of determining how the transition should be treated if the
> timezone is transposed to a different offset due to being used as the
> basis for interpreting POSIX rules.
> This data is in the sections which tzh_ttisgmtcnt/tzh_ttisstdcnt
> determines the size of.

Right.  Is this info actually used in practice?  If not, then I probably 
don't care.

Anyhow, back to my original question, I still don't understand why zic 
doesn't flag the Feb 9, 1942 transition as being observed at standard time.

Zone America/Atikokan    -6:06:28 - LMT    1895
             -6:00    Canada    C%sT    1940 Sep 29
             -6:00    1:00    CDT    1942 Feb  9  2:00s
             -6:00    Canada    C%sT    1945 Sep 30  2:00
             -5:00    -    EST

As part of my tzdist server, I've written code to translate iCalendar 
VTIMEZONE data back into tzfile format, and the only place that my files 
differ from files compiled with zic are those indicators in which an 
UNTIL time has a non-wall time suffix.  All my transition times and 
other indicators match.

For the zone above I have the ttisstd bit set in my data for the Feb 9, 
1942 transition.  zic (2015g) does not set this bit.

Kenneth Murchison
Principal Systems Software Engineer
Carnegie Mellon University

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