[tz] Non-valid timezones, is there a rule to remove them?

[Robert Elz]

    Date:        Thu, 23 Jan 2014 03:05:47 +0100
    From:        =?ISO-8859-1?Q?Fabiano_Fid=EAncio?= <fabiano at fidencio.org>
    Message-ID:  <CAK9pz9+4Gu8ziAH-eAkQPCRwYRzVqYAcsYrf7ynFiN9Xt_zhCA at mail.gmail.com>

  | Looking at the zone.tab file I can see a few timezones that, although valid
  | in the past, they are not valid anymore.

Sorry, that makes no sense.  How can a timezone ever become invalid?  We do
have old names, where a timezone has had the name we call it by changed - then
we keep the old name forever, for backward compatibility (possibly except for
a case where it existed such a short time with the wrong name that it would
probably never have been used - but I don't think that case has ever arisen.)

But the only way a timezone could ever become invalid would be if it had
never been valid in the first place.

  | I can pick as example, reported on
  | the GNOME Bugzilla[0], Asia/Novosibirsk, that used to be UTC+7, but
  | nowadays is considered the same one than Asia/Almaty, which one is UTC+6.

Unless Asia/Almaty was always the same as Asia/Novosibirsk and both changed
from UTC+7 to UTC+6 at the same time (having two identical zones is something
that isn't terribly useful, but has happened, and where it is found, we tend
to make one of them just be an alias for the other - just the same as if had
first been called one name and then the other).  But if that's not the case,
and I believe here it isn't, then how would you translate an old timestamp
from Asia/Novosibirsk (from when it was still UTC+7) if the rules you're using
are those for Asia/Almaty which is & was (at the relevant time) UTC+6 ?

  | How is the rule about keep or remove these not-used-anymore timezones?

They are never removed.  The bug is the "not used anymore", which is
the wrong thing to do, and most likely results in incorrect conversions.

  | Would you guys accept a patch removing the not-used-anymore ones?

I suspect not.

  | What kind of information should I provide/get to be sure that one timezone
  | could be removed?

You could find evidence that the data we have (historical data) is in fact
wrong, and that Asia/Novosibirsk and Asia/Almaty (for the example you gave,
the same would apply in any other case) have in fact always used the same
timezone, at least since 1970 (but the further back in time they have been
the same, the better) and that once corrected, the two zones would be
identical.  But even then neither will be completely removed, one might
just be made into an alias of the other (and no guarantee that would even
happen - it would also depend upon the projected likelihood of the two
cities remaining on the same timezone for the projected future.)

kre