[tz] Lack of "initial" transitions for some zones

[Jon Skeet]

On 21 July 2015 at 19:01, Lester Caine <lester at lsces.co.uk> wrote:

> On 21/07/15 18:47, Jon Skeet wrote:
> > However, the legacy "root" zones of CET, EET, PST8PDT etc don't have
> > this initial "transition" in the file - unless I'm misinterpreting it,
> > which is always a possibility.
>
> Since several areas merged to adopt these generic timezones, just which
> preceding time does one choose? The zone offset did not exist prior to
> it's adoption, and assuming any one offset prior to that is always wrong
> for other areas.


Well zdump already assumes an offset, in order to provide the local time
just before the first transition... and indeed zic does to work out when
the first transition occurs. So there's already some precedent for the
assumption.


> > Would it make sense to adjust either the data or zic to make all zic
> > output (in the 8-byte-timestamp format, at least) cover all of time? I
> > realize that this "extra" data (presumably an insertion of "standard
> > time until the first transition") may well not be historically accurate
> > - but I doubt that it would be any less accurate than other zones in a
> > similar situation.
>
> It will always be less accurate where several areas combined to adopt a
> new 'standard' time. The time prior to that may well be 'local mean
> time' and tagging THAT is accurate, but then selecting one LMT from
> several is always going to be wrong for the rest?


The zones in question all have a standard time and a daylight time. The
first transition is into daylight time. I would assume standard time before
that.

The zones in question are all just abbreviations, too - not actual
locations as such. To be less lazy than I was before, the complete list is:
CET, CST6CDT, EET, EST5EDT, MET, MST7MDT, PST8PDT, WET

The Theory file describes the US zones in that list as "legacy names" and
the European ones as "old-fashioned names". The europe file describes them
with: "These are for backward compatibility with older versions." I don't
know if that changes things at all.

Given the existing caveats about "don't trust that values before 1970 are
historically accurate" I think it's reasonable to simply assume standard
time before then, isn't it? Having said that, WET and EET are somewhat
interesting in that their first transitions are in 1977 - so after the
"accuracy watershed" so to speak. That does complicate things somewhat.

Jon
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