[tz] Issues with pre-1970 information in TZDB

Guy Harris gharris at sonic.net
Wed Sep 22 21:39:01 UTC 2021

On Sep 22, 2021, at 2:17 PM, Tom Lane <tgl at sss.pgh.pa.us> wrote:

> Guy Harris via tz <tz at iana.org> writes:
>> Presumably "stability" here includes "don't convert existing tzdb regions to links merely because they have the same 1970-and-later data" (putting them into the "don't merge" camp); does it also include "don't split existing tzdb regions due to the discovery that different parts of those regions had different pre-1970 data"?  (Those who want a full historical record of time zone data would presumably want those existing regions split.)
> For my own purposes, splits are not particularly problematic as long as
> the existing name can keep the existing historical data --- nobody is
> forced to adopt the new zone immediately, and whatever stored timestamps
> they have still mean the same thing.
> It gets a little more exciting if we discover that, say, Europe/Berlin's
> back data is more appropriate to some other place than it is to Berlin.
> OTOH, it's not clear how that differs from "Europe/Berlin's back data
> is wrong", so probably we'd just fix it and move on.
> In general, I think that incremental changes that clearly (or at least
> plausibly) improve the accuracy of tzdb's description of reality are
> fine.  One thing that's particularly sticking in my craw about the
> changes under debate is that they undeniably made tzdb worse as a
> description of reality in the places at issue.

There's "description of 1970-and-after reality" and there's "description of pre-1970 reality".

As far as I know, tzdb must accurately describe 1970-and-after reality, especially present-day-reality (otherwise a lot of its users will have to use their own fork that *does* accurately describe it), and if that requires that a region be split, so be it; anybody who wants "stability" in the sense of "never ever split regions" is best advised to spend their time lobbying governments not to take actions that would require region splitting rather than asking the tzdb maintainer not to split regions.

It's pre-1970 reality that's causing this issue (as the subject line of this thread indicates).

For those who, in Stephen's taxonomy:

> More broadly, different individuals, some representing organizations,
> have expressed different opinions on what they do or do not want from
> the tzdb data set. Some would like a full historical record of time
> zone data, others want stability, many I suspect have absolutely no
> interest whatsoever in pre-1970 data.

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.

For those who have no interest whatsoever in pre-1970 data, some might be "split or don't split, I don't care" and some might be "don't split", depending on the extent to which they care about stability in the sense of "no new regions unless required by 1970-and-later differences".  They presumably aren't bothered by the proposed merger, as it only affects pre-1970 data.

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"?

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