From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
cc: ysangkok@gmail.com, Pg Docs <pgsql-docs@lists.postgresql.org>
Subject: Re: Second-granular timezone offset format not documented
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Comments: In-reply-to Thomas Munro <thomas.munro@gmail.com>
	message dated "Mon, 05 Jul 2021 18:12:30 +1200"
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Date: Mon, 05 Jul 2021 10:04:27 -0400
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Thomas Munro <thomas.munro@gmail.com> writes:
> As for whether it's valid, that's coming from the IANA tz dataset.  It
> has a moment that it believes standard time to have begun at each
> location, in this case:
> Z America/Mexico_City -6:36:36 - LMT 1922 Ja 1 0:23:24
> https://en.wikipedia.org/wiki/Time_in_Mexico#History seems to agree on
> the year at least.  That "local mean time" offset is computed from the
> location's longitude, for lack of anything better.  The tzinfo
> "Theory" file has a bunch of disclaimers about pre-1970 data though,
> including "the tz database's LMT offsets should not be considered
> meaningful".

I tried to interest them in dropping the LMT idea altogether [1].
Unsurprisingly, the proposal went nowhere.

			regards, tom lane

[1] http://mm.icann.org/pipermail/tz/2021-May/030114.html