[tz] Issues with pre-1970 information in TZDB

[Robert Elz]

    Date:        Thu, 23 Sep 2021 07:49:31 +0200
    From:        Eliot Lear via tz <tz at iana.org>
    Message-ID:  <1949110b-0201-63a3-bb7f-728bc868a7f0 at lear.ch>

 | Unless of course if the data moved is questionable of questionable origin. 

The data may have 3 different (perhaps more if one cared enough) trust
levels...

1. It can be backed by verifiable sources that seem to be reliable, so
we believe it.

2. It is disputed by verifiable sources, that seem to be reliable,
so we consider it incorrect.

3. Neither of those, and we simply have no way to know.

In that, no-one questions use of type 1 data, and type 2 simply becomes
type 1 after the obvious correction is made.

The disputes are all about #3 - and certainly you can call that questionable,
or arguable, or various other similar terms - none of which actually mean
anything.   Everything can be questioned, and so is questionable.  What the
time currently is, right now, in London can be questioned, so that is
questionable.  Should that data be removed from tzdb because of that?

If you want to claim some data is incorrect, provide evidence to support
that, otherwise you're simply maligning whoever supplied the data in the
first place (nb: not necessarily the person who edited it into tzdb, but
the source of their information.)

For any data to be in tzdb, at some time or other, someone believed it
might have been correct (no-one has ever, typos excepted, deliberately added
bogus data) - their belief might have been incorrect, but to remove that
data, just wondering "was that correct" should not be enough - we should
need #2 type evidence - something that shows that some entry is actually
wrong.   That has happened often enough that we know corrections get made
when needed.  Simply discarding data because we neither ever had (or perhaps
simply did not record, perhaps because it was not provided to us) the
evidence to support some apparent claim is not a sane way forward.

And it is even worse when the effect of this is to (effectively) remove
the old data (anything present in backzone is effectively removed, even
though it is still there in the file, as no-one uses it).  Eg: to re-use
an example that has been used here a bit recently, if someone today looks
at the app which translates dates & times of historic events to the local
time of the requestor ("what time was it, for me, when Armstrong stepped
onto the moon's surface?" etc).   If someone in Montreal did that, and got
what they believe was the wrong answer (because they remember the event),
then investigate why, and see "We use the data for Toronto, and that answer
was correct for Toronto" they'd perhaps wonder why, but that would be the
end of it.   But if they were instead using the data for Montreal, and it
turned out to be incorrect, they'd actually complain (probably to the
authors of the app, who would then complain here).   That way the data
might be corrected.   But not if it is buried in backzone.

kre