[tz] POSIX and Palm Springs vs. Ireland

Steve Allen sla at ucolick.org
Fri Jan 19 17:29:43 UTC 2018

On Fri 2018-01-19T14:22:18+0700 Robert Elz hath writ:
> More than that, as there are (for purpose 1 above) just two values
> possible (0 and 1) and there is no such limit on the number of
> different abbreviations which might apply at different times in
> one of what we call a timezone (that is, a juristiction might decide
> to have winter time, spring time, summer time, and autumn time, all
> with different offsets, and give them all different abbreviations),
> so we really should be deprecating tm_isdst for even this purpose, and
> rely upon tm_zone and tm_gmtoff instead, which are a much better way
> to pass the required info out from localtime ... any implementations
> which are yet to add those fields to struct tm (they were created
> decades ago - there really is no excuse after all this time) should
> simply be regarded as broken and unusable.

The original POSIX draft was IEEE Std 1003.1 issued for Trial-Use in
April 1986, and the final version was IEEE Standard Portable Operating
System Interface for Computer Environments (IEEE Std 1003.1-1988) in
September 1988.  Those make it quite clear that the POSIX committee
included the System V tm_isdst merely to accommodate its ongoing use
even though that scheme was hard-coded to apply only to US usage.
It is a dark moment in history that the POSIX committee did not
wholeheartedly support the tz project that already then existed.

Palm Springs California is another example where that simple model was
always broken.  The shadow of Mt.  San Jacinto brings darkness very
early in the winter months.  In 1946 the chamber of commerce decided
to put the clocks of Palm Springs forward by an hour in the winter.


Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
UCO/Lick Observatory--ISB 260  Natural Sciences II, Room 165  Lat  +36.99855
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Santa Cruz, CA 95064           http://www.ucolick.org/~sla/   Hgt +250 m

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