[tz] Preparing to fork tzdb

Guy Harris gharris at sonic.net
Fri Sep 24 19:34:42 UTC 2021

On Sep 24, 2021, at 8:05 AM, Clive D.W. Feather via tz <tz at iana.org> wrote:

> 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.

The further back you go from 1970, the more likely that the data for the tzdb region you're in - even if all of backzone is included - is likely to be wrong for your current location.

Fixing that would probably require a lot of detailed research, at least for some locations and, even if you don't try to give LMT for every location on earth, could involve creating significantly more tzdb regions.  And the further you go back, the more research and the more tzdb regions will be necessary.

So if the tzdb were to make an effort to improve the correctness of pre-1970 data over and above undoing all the moves to backzone, the cost - in effort required to get better data for existing regions, effort required to get data for any newly-created regions, effort required to maintain those new regions, *and* effort by at least some consumers of the to use the newly-created regions.

It may be that the best pre-1970 data you'll ever get from the tzdb will be data for existing regions, including backzone regions, with whatever fixes are supplied by those interested in improving that data.  You may never get, for example, finer-grained data for Germany than you have now, if there is any.

So, yes, moving regions to backzone may just be replacing incorrect data with incorrect data, although the data moved to backzone might be "less wrong", if that matters to consumers of pre-1970 data.

(BTW, macOS uses at least two relegated-to-backzone tzids - if I set my current city to Montreal, it sets the current tzdb region to America/Montreal, not America/Toronto, and if I set my current city to Asmara, it sets the current tzdb region to Africa/Asmara, not Africa/Nairobi.  However, they don't appear to use the backzone data, as the America/Montreal and America/Toronto files are byte-for-byte identical, as are the Africa/Asmara and Africa/Nairobi files.  I don't know whether, if I were to travel to Montreal or Asmara, they'd set the tzdb region to America/Montreal or Africa/Asmara, respectively.  That depends on the map data they use, and whether it has those regions.)

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