The “should timezones be merged when they contain identical offsets from 1970 onwards” discussion has been going on for quite a while on the tz mailing list. Still, contributions that I find insightful are still being made. Stephen Colebourne recently made this observation:
The key observation is that the segregation between these two groups
of IDs *did not exist* until around 2014. It was only in 2018 that the ISO country rule was removed.
This change was reflected in the documentation <theory.html> as recently as 20/21 February 2019 < https://github.com/eggert/tz/commit/5f48dee82bc111b21f3610893e3b788e37396bce
< https://github.com/eggert/tz/commit/6176aefe79e83ddb8f255849b85c149f34d46aba
And Stephen made this spot-on point of analysis:
FWIW, it is clear to me that there is an aspect of imposing a
US-centric timezone system on other parts of the world. The recent tzdb approach of focussing entirely on timezone regions makes perfect sense for the US, where region boundaries do not follow state lines, and ordinary members of the public need to be aware of whether they are in US/Mountain or US/Central. This simply isn't the timezone model in many other parts of the world. In places like Europe and Asia, the timezone is driven primarily by the country you live in - an ordinary member of the public in Iceland is never going to associate with some abstract timezone region stretching down the Atlantic that is not named, not legally defined and is little more than a random outcome based on tzdb's choice of 1970. Even in somewhere like Norway, an ordinary member of the public will understand that although they follow CET, their timezone is actually driven by their Government in Oslo. The brilliance of the original rule - "ID as needed for post-1970 data, with at least one per ISO country" - was that it seamlessly handled *both* models of timezone in one unified set of IDs. Removal of the ISO country part has completely destabilised that balance.
If I may throw in my two-penny, it would pertain to the documentation of this policy. Presently, <theory.html> does mention - that timezones are zones whose clocks all agree about timestamps that occur after the 1970 POSIX Epoch; - that 1970 is indeed a somewhat-arbitrary cutoff, but that moving it earlier even by a decade or two would impose significant challenges, due to the wide variety of local practices before computer timekeeping became prevalent; - that pre-1970 clock transitions are recorded for location-based timezones, but that the database is not designed for and does not suffice for applications requiring accurate handling of all past times everywhere; - that the previous “at-least-one-timezone-per-country-with-officially-designated-ISO-3166-1-two-letter-country-code” rule is dropped for this imposed too high a maintenance burden; and - that some information outside the scope of the database (implied: pre-1970) is collected in a file backzone that is distributed along with the database proper, but that this file is less reliable and does not necessarily follow database guidelines. What astonishes me in this issue is that high-quality information is being removed from the database proper into backzone, without this being mentioned in the documentation <theory.html>. Granted, <theory.html> does not explicitly state an “at-least-attempt-to-get-monotonically-closer-to-the-truth-with-increasing-versions” policy, but one would think that’s implied in maintaining a (versioned) database, unless explicitly stated otherwise. The other way around (moving information from backzone to the database proper, e.g. when historical information is double-checked and confirmed to be in accordance with database guidelines) is totally acceptable without mentioning in the documentation <theory.html>. Indeed, even though I know that correcting pre-1970 data is not the priority of this project, I’ve seen some great sluthing of historical time zone offsets presented on the tz mailing list in the past, and I kinda assumed that the results of such research found its way to the database proper (or backzone when doubts remained). In my opinion, removing high-quality information — thereby having correct offsets changed into incorrect offsets in information systems that rely on the tz database and keep it up-to-date on the default settings (which disregard backzone) — should cease, and should preferably be undone where such damage has already occured. Seeing how much problems pursuing “fairness” under the new rules causes, perhaps it’s worth reinstating the previous “at-least-one-timezone-per-country-with-officially-designated-ISO-3166-1-two-letter-country-code” rule, either officially or unofficially. (And for what it’s worth, this comes from someone who generally tries to avoid relying on country lists such as ISO 3166-1, due to strong political belief in the desirability of independence or at least increased self-governance for several countries/regions currently lacking an officially-designated ISO 3166-1 two-letter country code, including Flanders, Wallonia, Scotland, Catalonia, Basque Country, Canary Islands, Tibet, and others.) But alternatively, if this project persists in its “removing-high-quality-information and merging-for-merging’s-sake” policy, then at the very least this should be mentioned clearly, unambiguously and conspicuously highlighted (bold, bigger font size, bright but fully saturated background colour, following an animated warning icon, the lot) in the documentation <theory.html>: PREVIOUSLY OR CURRENTLY CORRECT PRE-1970 OFFSET INFORMATION IS
INTENTIONALLY BEING CHANGED AND/OR MAY INTENTIONALLY BE CHANGED IN LATER VERSIONS OF THE tz DATABASE INTO INCORRECT OFFSET INFORMATION for the braindead reason of merging timezones for merging’s sake. USERS MUST NOT RELY ON UNDISPUTEDLY AND UNCONTESTEDLY CORRECT AND VERIFIED OFFSET INFORMATION REMAINING CORRECT in the database proper, except for timezones that have post-1970 particularities which prevent them from being merged with foreign timezones. (And even then — who knows what the coordinator-maintainer will come up with next…) Information systems that rely on tz for offset information, including for pre-1970 datetimes-with-timezone-identifier, ought to pass this warning on to their users.
— Adhemar