[tz] Preparing to fork tzdb

Robert Elz kre at munnari.OZ.AU
Fri Sep 24 18:53:20 UTC 2021

    Date:        Fri, 24 Sep 2021 16:05:45 +0100
    From:        "Clive D.W. Feather via tz" <tz at iana.org>
    Message-ID:  <YU3pSS/8EM0Nqjkd at davros.org>

  | I see that point, though it depends on how you view pre-1970 data. After
  | all, the data is already wrong for much of the area covered by
  | Europe/Berlin and possibly (I wouldn't be surprised either way) much of
  | the area covered by Europe/Oslo.

It isn't "because they're different countries" but because there was
data that is now (effectively) deleted (for anyone who doesn't make a
private fork - and I don't think anyone really believes that any kind
of fork of the data is a good idea).

In an earlier message, Paul said:
  | so far it's been very unlikely,  for example, that Sweden would diverge
  | from Germany. (This may of course  change in the future, in which case
  | we'll of course deal with it.)

"deal with it" means, of course, change Europe/Stockholm back from
being a Link (assuming it ever actually becomes one) to being a Zone

In other words, any authority with the ability to make timezone changes
who wants to have their own zone, with their own data (including as much
of their own historical data as they can manage to collect) simply needs
to change their timezone in some small way, and we have no choice but to
give them a zone, after which they can submit the historical (pre 1970)
data, and it will be included.   They cannot get that in how, as historic
data for Sweden is probably not going to be correct for Berlin.

This is not something we ought to be encouraging - authorities already
make bizarre changes for no seemingly good reasons (to outsiders anyway)
we should not be giving them another reason to make even weirder changes,
just to satisfy some "rule" that we have imposed.

To the extent that we're concerned that even though Berlin is the
correct choice for Germany according to the rules, its time history
is not accurate for much of the rest of Germany, then there's nothing
stopping us treating this as a somewhat special case (which it clearly is),
and creating Europe/Fankfurt (or Hamburg, Munich, or ..., I have no idea
which city would qualify) to contain what would be a better representation
of the data for the bulk of what used to be West Germany.  Which one
people used for post 1970 timestamps wouldn't matter, but people who want
more accurate older data can select which is appropriate.   If Berlin is
also not accurate for Leipzig (and Frankfurt or whatever isn't either) then
we could also create a zone for what used to be East Germany, to contain
the historic data for them.

Someone would need to do the work to collect the data first of course,
just saying "it should happen" isn't enough to cause that to occur.
Someone who needs it needs to do the work first.

That the data doesn't differ post 1970 does not exclude a new zone, it
just does not require that one be created, there's a discretion.  Anyone
who believes that ever was more than a guideline just needs to look at
the zones that are being, and have previously been, merged.  If that
post 1970 "rule" was a requirement, there would never have been all these
post 1970 identical zones to merge.   If there's a good enough reason,
we can create multiple zones that are identical post 1970, and what's more,
we should.

Of course, there's some discretion involved - decisions need to be made in
each case, rather than simply blindly following some rule, but that's
what rational people do - weigh the cost/benefit and decide case by case.


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