[tz] McMurdo/South Pole

Guy Harris guy at alum.mit.edu
Sun Sep 22 18:30:13 UTC 2013

On Sep 22, 2013, at 5:17 AM, Stephen Colebourne <scolebourne at joda.org> wrote:

> On 22 September 2013 08:43, Guy Harris <guy at alum.mit.edu> wrote:
>> If you care about getting the *right* answer, the only concerns should be about cases where the current pre-1970 data is known to be correct and the changes will eliminate correct data.  Discarding data not known to be correct could just replace one incorrect answer with another.
>> If you only care about getting *an* answer, it obviously doesn't matter.
> I don't just want *an* answer, and I'm not obsessed by the *right*
> answer, my problem here is a *wrong* answer (any clock change
> including DST before 1956 is wrong).

OK, so, in that particular case, given that it would be misleading to say DST was in effect or that the time was some form of New Zealand time prior to the establishment of the base, I'd be inclined to say that the old entry for McMurdo Sound should perhaps be re-instated, complete with "zzz".

As for the general case, the "Scope of the tz database" section of the Theory file says

	The tz database attempts to record the history and predicted future of all computer-based clocks that track civil time.

The Wikipedia article for "civil time" says

	In modern usage, civil time refers to statutory time scales designated by civilian authorities, or to local time indicated by clocks. Modern civil time is generally standard timein a time zone at a fixed offset from Coordinated Universal Time (UTC) or from Greenwich Mean Time (GMT), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks, and was adopted in 1972. Older systems use telescope observations.

	In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity[clarification needed], whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight, but the two Nautical Almanac Offices did not accept it until 1952.

which doesn't give a definitive answer as to whether "civil time" has to refer to "statutory time scales designated by civilian authorities".

My inclination would be to explicitly state that it *does* refer to statutory time scales, other than local mean time, designated by a government authority, i.e. that, prior to the establishment in law of a time scale other than "use local mean time for *your* location", the tzdb doesn't attempt to record anything - it only records the point in time at which that standardized time scale was first established.

I'd then have the first Zone line for a tzdb zone give, as the Until column, the point in time at which the standardized time scale was first established.  I don't strongly care what time offset or zone abbreviation is chosen for them; my personal choice would be to use the initially established standardized time offset and time zone abbreviation (so as to project standardized time infinitely far back into the past).

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