[tz] Fwd: DST changes in Hungary (full historical revision)

Michael H Deckers michael.h.deckers at googlemail.com
Thu Jun 11 11:59:57 UTC 2020

     On 2020-06-10 21:25, Paul Eggert wrote:

> Anyway, the bottom line is that the stopped clocks make this an oddball
> transition that cannot be modeled exactly by tzdb, and that given the story you
> mentioned you are correct that it's better modeled using an ordinary transition
> (from 00:09:21 to 00:00:00) than the unusual transition we're currently using.

     Let us not get carried away and consider this an oddball transition.
     It was a transition from a local time to a time zone time that is just
     a few minutes slower, as has happened in many locations all over 
the world.
     Legal time in France was never stopped; it just switched from UT + 
     to UT.

     Since 1891-03-16 (sic!), legal time in France was the mean solar
     time in Paris, and the law published on 1911-03-10 (online at
     [https://gallica.bnf.fr/ark:/12148/bpt6k2022333z/f2]) changed it
     to mean solar time in Paris minus 00:09:21; this change took effect
     on the day after publication, 1911-03-11. The appended decree
     (loc cit) makes it clear that the new definition of legal time
     was used with values from 1911-03-11T00:00:00 onwards. There is
     no mention in the legal texts of how the change is to be effected
     (let alone that legal time would stop for some time).

     Hence, the old definition of legal time applied until just before
     it took the value 1911-03-11T00:09:21, and the jump in legal time
     was from 1911-03-11T00:09:21 to 1911-03-11T00:00:00.

     The newspaper reports about some remote-controlled public clocks
     ("pendules pneumatiques") shows that these clocks were stopped
     when they reached 1911-03-11T00:00; the next minute pulse only
     came 10 minutes and 21 seconds later and put them to 1911-03-11T00:01.
     That does not imply that legal time had stopped, it just indicates
     how these clocks were adjusted. Other clocks (such as clocks on
     churches and pocket watches) would certainly have been adjusted
     in a less time-consuming manner.

     Railway time in France was also adjusted at the same occasion
     from UT + 00:04:21 to UT. Here it may be more appropriate to
     say that "time was stopped" (at least for the trains not
     currently in motion) because railway time is the parameter
     used for planning train motions and should be monotone.

     Michael Deckers.

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