On Mon, 4 May 2020 at 17:43, Jeff King <peff@xxxxxxxx> 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
