[tz] Issues with pre-1970 information in TZDB

Tom Lane tgl at sss.pgh.pa.us
Thu Sep 23 03:24:46 UTC 2021

Guy Harris <gharris at sonic.net> writes:
> I sent a message to the list noting that:

> 	stuff has been moved to backzone in the past;

> 	the backzone comment in 2021a speaks of at least some of the data there being questionable;

> 	as an example, America/Montreal, moved to backzone in 2015 or so, has comments suggesting it's not reliable, while Europe/Oslo's comment seems to suggests that the original sources agree with The Norwegian Meteorological Institute and thus that the data is presumably considered reliable.

> Hopefully useful discussion of the "considered-to-be-reliable pre-1970 data vs. not-considered-to-be-relaible pre-1970 data" question will ensue.

I just spent awhile looking through backzone as of 2021a, and the stuff
that was added to it in May.

Nearly all of the older entries have either no documentation at all, or
comments explicitly questioning their veracity.  I count 82 non-comment
Zone lines in the 2021a data, of which perhaps three have enough positive
supporting commentary that maybe there's a case for promoting them into
the default zone set.

The May patches moved 32 new Zones into backzone.  These zones are,
as far as I can see, mostly *far* better attested than what was there
before.  There are definitely a few that belong in backzone if the
standard is "do we trust this data", but it's hard to avoid the
conclusion that a double standard has been applied here.

Don't take my word for it; look through the files for yourself.

			regards, tom lane

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