Re: :format:%s for date fields seems to be shifted by timezone

>
> 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)".