[tz] Preparing to fork tzdb

Kevin Kenny kevin.b.kenny at gmail.com
Thu Sep 23 13:16:23 UTC 2021

On Thu, Sep 23, 2021 at 8:30 AM Clive D.W. Feather via tz <tz at iana.org>

> Therefore any argument that they should be two or three zones instead needs
> to be based on some *other* objective requirement. Which, I suspect, needs
> to be *added* to the present policies rather than just handwaved. Such
> requirements could be:
> * Never abolish a zone once it's got into the default setup.

I could live with this. My chief concerns are (a) we should not arbitrarily
make data that are believed to be reliable more difficult to access; (b) we
should not surprise users by suddenly making them see different names for
their home time zones. (For a Norwegian suddenly to have 'Berlin' instead
of 'Oslo' appear is not acceptable.) Could we establish the rule, "don't
abolish zones once released, even if they are subsequently discovered to be

I understand the desire to get rid of data that are known to be incorrect
or at least unreliable. I have no objection, for instance, to removing
pre-1970 data from Montréal if it appears that Shanks compiled incorrect
information. While that might change the interpretation of historical dates
to be a proleptic version of whatever the situation was in 1970, that at
least keeps things compatible, and is no different in effect from a
correction to the historical information.

> * At least one zone per 3166 "country".

I don't see this as necessary, but it's Mostly Harmless. Links are cheap.
It still would run afoul of similar problems to the one we are facing,
because it's entirely possible that a zone with deleted data will turn out
to be in a country with multiple zones.

> * At least one zone per "authority".

This one is almost certainly unworkable, since it puts whoever maintains
tzdb in the position of determining who is a competent authority. We have
known cases where a region with an ethnic, religious or political minority
maintains a different time zone in defiance of a central authority, or
where there is a time zone maintained by a government whose legitimacy is
disputed.  Moreover, there is a definite problem with multiplication of
authorities (see below).

> * Use separate zones if we have reliable data going back to 1945 (or pick
> some other year).

>From a US-centric perspective, the choice of 1970 was not exactly an
accident. If you go back much farther, you run into tremendous
Balkanization of local authorities.  I can recall one archivist that I knew
regaling me with the story of the time in the late 1960's that he had to
endure no fewer than seven time zone changes in the course of a 45-minute
bus journey, because individual towns and counties in West Virginia and
Ohio had decreed different dates for the Daylight Saving Time transition.
The date could readily be pushed back to 1967. Prior to that, what we would
have would be valid only for individual municipalities; :America/New_York
would not have the same rules as a hypothetical
:America/West_Virginia/Charleston. (Charleston in turn would not have the
same rules as Wheeling, Moundsville or Morgantown. It was an unbelievable
mess back then.

73 de ke9tv/2, Kevin
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