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. https://www.desertsun.com/story/life/2017/12/27/palm-springs-struggle-daylig... -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m