[tz] Issues with pre-1970 information in TZDB
gharris at sonic.net
Wed Sep 22 22:32:19 UTC 2021
On Sep 22, 2021, at 3:06 PM, Tom Lane <tgl at sss.pgh.pa.us> wrote:
> 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.
Currently, people who need a full historical record of time zone data had better acquaint themselves with how tzdb is set up on the system on which they're using it, as that affects whether they have a full historical record of time zone data - if the data they're using doesn't include backzone, there are a number of tzdb regions for which it does *not* include a full historical record of time zone data, e.g. Montreal.
They'd also better look at the tzdb source files to see where there is pre-1970 data suspected of not being correct - again, see Montreal:
# From Paul Eggert (2015-03-24):
# Since 1970 most of Quebec has been like Toronto; see
# America/Toronto. However, earlier versions of the tz database
# mistakenly relied on data from Shanks & Pottenger saying that Quebec
# differed from Ontario after 1970, and the following rules and zone
# were created for most of Quebec from the incorrect Shanks &
# Pottenger data. The post-1970 entries have been corrected, but the
# pre-1970 entries are unchecked and probably have errors.
I.e., people in the "need a full historical record of time zone data" group are advised not to take an "I'll just trust that somebody's set up the data in the way that I need and have vetted it" approach.
Those in the "would like a full historical record of time zone data" but aren't in the "need a full historical record of time zone data" should consider why they would like that full historical record and either 1) put themselves in the "need a full historical record of time zone data" group and do the necessary work to ensure that they have that record or 2) arrange to work without a full historical record of time zone data.
> I will stipulate that if you'd been using backzone all along, you'd be
> unaffected by the May changes.
I.e., anybody in the "need a full historical record of time zone data" who has a clue will be unaffected.
Those *in that group* (i.e, the *need* group) who do *not* have a clue need to be given one, so that they do the necessary work.
> The problem is that a lot of people who
> never heard of that, and have no input into its use, will be affected.
OK, so which group(s) are those people in?
Those in the "need a full historical record of time zone data" group, see above.
Those in the "a full historical record of time zone data is nice to have, but not necessary" should decide whether they belong in that group, and what that means, or just decide that they're in the "stability is important" group or the "I don't really care about pre-1970 data so, if it gets merged, that's not a real problem" group.
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