[tz] Issues with pre-1970 information in TZDB

Tom Lane tgl at sss.pgh.pa.us
Wed Sep 22 22:06:34 UTC 2021

Guy Harris <gharris at sonic.net> writes:
> For those who, in Stephen's taxonomy: ...
> would like a full historical record of time zone data, their rules would presumably be "if we discover that a given tzdb region didn't uniformly have the same offset or rules all the way back to the establishment of standard time, we should split it".  They are presumably using backzone, as, without it, you don't have a full historical record of time zone data, and would thus be unaffected by the proposed merger.

I don't think that last statement follows from the available information.
People who are not intimately familiar with how tzdb is set up won't
even know that backzone exists.  Many of the consumers who might have a
stake in this discussion don't have any option there anyway, because
they are using a tzdb distribution made by somebody else.

I will stipulate that if you'd been using backzone all along, you'd be
unaffected by the May changes.  The problem is that a lot of people who
never heard of that, and have no input into its use, will be affected.
I'm not convinced that the existing users of backzone are the people to
optimize this change for.

> For those who want stability, they are, presumably, those who *do* care about pre-1970 data, so they presumably don't want regions to disappear in favor of links due to one region sharing 1970-and-later data with another region, and are thus opposed to the proposed merger.  Do any of them also want "no new regions unless required by 1970-and-later differences"?

As you pointed out yourself, all consumers of tzdb had better be prepared
for zone splits, because one could be forced on them at any time due to
governmental decisions.  I doubt there are people for whom a split due
to ancient history is worse than a split due to current changes.

			regards, tom lane

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