Re: git-2.13.0: log --date=format:%z not working

From: Ulrich Mueller <ulm@gentoo.org>
Date: Jun 2, 2017, 22:04 UTC
Message-ID: <22833.57584.108133.30274@a1i15.kph.uni-mainz.de>
In-Reply-To: <20170602183504.ii7arq2ssxgwgyxr@sigill.intra.peff.net>

>>>>> On Fri, 2 Jun 2017, Jeff King wrote:

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> On Fri, Jun 02, 2017 at 07:25:43PM +0200, René Scharfe wrote:
>> On Linux "%z %Z" is expanded to "+0200 CEST" for me, while on Windows I
>> get "Mitteleurop▒ische Sommerzeit Mitteleurop▒ische Sommerzeit".  (That
>> "▒" is probably supposed to be an "ä".)  POSIX requires  +hhmm or -hhmm
>> format for %z, and for %Z is to be "Replaced by the timezone name or
>> abbreviation".

Actually, the POSIX definition for %Z continues: "or by no bytes if no timezone information exists." So also returning an empty string would be compliant (but maybe not very helpful).

>> I'd say "GMT+0200" etc. is a nice enough timezone name, i.e. having %Z
>> resolve to the same as %z plus a literal prefix of "GMT" should at least
>> not be wrong.

> I thought that, too, but I think it is wrong based on my understanding
> of how $TZ is parsed. There something like "EDT-4" means "call this EDT,
> and by the way it is 4 hours behind GMT".

> So what you're proposing isn't wrong per se, but your notation means
> something totally different than what similar-looking notation looks
> like on the $TZ end, which is bound to create confusion.

I agree that GMT+0200 could be misleading. But what about resolving %Z the same as %z in the case of the author's time zone, as was suggested earlier? It is supposed to be human-readable output, or do we expect that someone would use the %Z output and e.g. plug it back into their TZ?

>> Alternatively we could have a lookup table mapping a few typical offsets
>> to timezone names, but e.g. handling daylight saving times would
>> probably be too hard (when did that part of the world switch in the
>> given year?  north or south of the equator?)..

IMHO maintaining such a local table of timezones won't fly.

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> Right, I don't think the mapping of zone to offset is reversible,
> because many zones map to the same offset. If I tell you I'm in -0500,
> even just in the US that could mean Eastern Standard Time (winter, no
> DST) or Central Daylight Time (summer, DST). Not to mention that other
> political entities in the same longitude have their own zones which do
> DST at different times (or were even established as zones at different
> times; historical dates need to use the zones as they were at that
> time).

Same here, my +0200 offset could be anything of CAT, CEST, EET, IST, SAST, or WAST, according to IANA timezone data. It's a one-directional mapping, and there's no way to get the author's /etc/localtime info (or whatever its equivalent is on other systems) back from the offset stored in the commit. A timezone name may not even exist at all for a given [+-]hhmm offset.

Ulrich