[tz] Preparing to fork tzdb

[Phake Nick]

The current tzdb only have separate zone for data up to 1970, other than
some exceptions
Forking to include data from earlier times need to considering willingness
of general data user to switch over with a larger-sized database and less
stable[due to foreseeable upcoming contribution to historical timezone] as
well as less accurate data from the past, especially when the number of
computer applications and OS in modern time need to handle files with
timestamps before 1970 is probably relatively less.
It was not without precedent that data that were same after 1970 being
moved away from the main tz file in tz db, with examples including Chinese
timezones except Xinjiang, as well as Japanese Southwestern Islands
timezone.

在 2021年9月21日週二 13:43，Guy Harris via tz <tz at iana.org> 寫道：

> On Sep 20, 2021, at 7:57 PM, dpatte via tz <tz at iana.org> wrote:
>
> > When I read an astronomical journal and read of a bolide sighting in
> 1957 at 7:30 PM in haiti, it is important that I can attach that to a ut in
> 1957. But if it's now in the 'merged data' I am now out of luck.
>
> As long as it's well established that Haiti didn't observe DST in 1957, as
> the current northamerica file claims is the case, that works.
>
> If it's not, well....
>
> (I.e., how "reliable" data for a given tzdb region and a given time in the
> past is probably depends on how far back in the past it is and how well
> offset and time-shifting rule changes are documented - and what sources
> people have checked.)
>
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