[tz] tzdata isstd question

Brian Inglis Brian.Inglis at systematicsw.ab.ca
Fri Oct 30 21:33:30 UTC 2015

On 2015-10-30 11:52, Ken Murchison wrote:
> 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.

Reformatting lightly and adding headings and emphasis, we can see that in 1942, that zone and rule both say that time zone observed saving time: War time == DST - standard time was not resumed until Sep 1945.

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

Rule	Canada	1942	only	-	Feb	 9	2:00	*1:00*	W # War
Rule	Canada	1945	only	-	Aug	14	23:00u	1:00	P # Peace
Rule	Canada	1945	only	-	Sep	30	2:00	0	S

The US rules say the same as above, although practice may have varied depending on whether the zones normally observed DST.
The UK rules (GB-Eire) are similar, but summer time was observed from Feb 1940, and double summer time during the normal DST periods, only returning to GMT in Oct 1945.
Similar practices may have been observed in other allied countries while unoccupied during WWII.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

More information about the tz mailing list