From: clime <clime7@gmail.com>
To: Jeff King <peff@peff.net>
Cc: Git List <git@vger.kernel.org>
Subject: Re: :format:%s for date fields seems to be shifted by timezone
Date: Wed, 6 May 2020 20:03:52 +0200 [thread overview]
Message-ID: <CAGqZTUs+xCPjQdeQVfU6EWARmZdjcgcHeSNxkKaTf316rX+8pg@mail.gmail.com> (raw)
In-Reply-To: <20200504154343.GA12842@coredump.intra.peff.net>
On Mon, 4 May 2020 at 17:43, Jeff King <peff@peff.net> wrote:
>
> On Sun, May 03, 2020 at 12:15:14PM +0200, clime wrote:
>
> > my current timezone is UTC+0200.
> >
> > I create a test repo, add one commit and create a tag:
> >
> > Now:
> >
> > $ git for-each-ref --format="%(taggerdate:format:%s)" refs/tags
> > 1588504146
> >
> > $ date +"%s"
> > 1588500585
> >
> > $ git for-each-ref --format="%(taggerdate:raw)" refs/tags
> > 1588500546 +0200
> >
> > Somehow %(taggerdate:format:%s) gives a Unix timestamp which is one
> > hour in future and it is different than what ` date +"%s"` gives
> > around approximately the same time the tag was created.
>
> It's caused by strftime() being clever with DST. Try this:
>
> git commit --date=@1559361600 --allow-empty -m summer
> git commit --date=@1577854800 --allow-empty -m winter
> git log --format=%ad --date=unix >unix
> git log --format=%ad --date=format:%s >strftime
> diff -u unix strftime
>
> We get the winter time right, but the summer time wrong.
>
> The issue is that strftime() takes a broken-down "struct tm", not a unix
> time_t. We have all of the right values for hour/minute/etc there, so
> using "format:%H:%M:%S" prints what you'd expect. But we never set the
> "isdst" field, so when it tries to convert back to unix time, it applies
> a one-hour offset (if it's "summer" in your local timezone).
>
> Unfortunately I don't think we can solve this easily. If we were
> operating completely in your local timezone, then we would have gotten
> that "struct tm" from localtime(), and its isdst field would be set
> properly. And indeed, if you use "--date=format-local:%s", the problem
> goes away.
>
> But when we're formatting in the original author's timezone, which is
> the default, we have no idea if they were in dst then or not. We only
> know their offset-to-gmt, so we munge the time_t ourselves and use
> gmtime().
>
> So there are a few reasons I think this is the best we can do:
>
> - the full timezone information literally isn't there in Git; we might
> know the author was in +0200, but we don't know if they were
> observing DST, or if they were simply in a timezone further east.
>
> - even if we had a zone, there's no system function to convert a time_t
> to a tm in an arbitrary timezone (hence the gmtime() hack above;
> we've tried playing games with $TZ and tzset(), but it's awkward and
> unportable)
>
> - likewise, strftime() is doing the reverse conversion using the local
> timezone anyway, which would be wrong.
>
> So my advice is not to use "%s" (which isn't portable anyway). Use
> "--date=unix" or "%(taggerdate:unix)".
Hello Jeff,
what about just printing the raw timestamp from either commit or tag,
i.e. avoiding any conversion for format:%s?
Best regards!
clime
>
> -Peff
next prev parent reply other threads:[~2020-05-06 18:04 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-03 10:15 :format:%s for date fields seems to be shifted by timezone clime
2020-05-04 15:43 ` Jeff King
2020-05-06 18:03 ` clime [this message]
2020-05-06 19:06 ` Jeff King
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAGqZTUs+xCPjQdeQVfU6EWARmZdjcgcHeSNxkKaTf316rX+8pg@mail.gmail.com \
--to=clime7@gmail.com \
--cc=git@vger.kernel.org \
--cc=peff@peff.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).