Re: Time zone trouble without time zones being involved

[Yuri Khan <yuri.v.khan@gmail.com>]

On Sun, May 15, 2016 at 12:53 AM, Michael Heerdegen
<michael_heerdegen@web.de> wrote:

> Here are the results.  It is 20:44 here now after I run the tests
> (Central European Daylight savings time, the time zone of Berlin).

So, UTC+1 in winter, UTC+2 in summer.

>> 1. Mark the current UTC time, both as a human-readable string and as a
>> unixtime value (e.g. on GNU, “date --utc --rfc-3339=seconds” and “date
>> +%s”, respectively).
>
> date --utc --rfc-3339=seconds => 2016-05-14 18:38:17+00:00
> date +%s                      => 1463251129

The unixtime works out to 2016-05-14 18:38:49+00:00, consistent with
the UTC time. Also consistent with your wall clock considering the
UTC+2 offset.

>> 2. Make a commit.  3. Execute the same command, “git log
>> --pretty=format:%ad --date=format:%s -n1”.
>
> 1463254759

1463254759 - 1463251129 = 3630 seconds, or 1 hour 30 seconds.

Does “git cat-file commit HEAD” show the same number in the “author” line?

>> 4. Evaluate (current-time) and convert the first two elements of the
>> resulting 4-element list to unixtime.
>
> (let ((time (current-time))) (+ (* 65536 (car time)) (cadr time)))
> ==> 1463251305

176 seconds from step 1.

> Looks like git is 1 hour wrong...?

Yes.


Here’s a repeatable recipe. You can report it as a Git bug.

0. Versions

    $ git --version
    git version 2.8.2

    $ dpkg -l tzdata | tail -1
    ii  tzdata  2016d-0ubuntu0.16.04 all  time zone and
daylight-saving time data

1. Initialize an empty Git repository:

    $ git init test
    $ cd test

2. Make a commit, using the Europe/Berlin time zone:

    $ TZ=Europe/Berlin git commit -m 'test' --allow-empty

3. Examine the timestamp recorded in the commit object:

    $ git cat-file -p HEAD | grep author
    author Yuri Khan <yurivkhan@gmail.com> 1463260938 +0200

4. Check that it corresponds to the current time:

    $ date +%s
    1463260977

5. Try to get the commit date in the unixtime format:

    $ TZ=Europe/Berlin git log --pretty=format:%ad --date=format:%s -1

Expected result: 1463260938 (same as recorded in the commit object).
Observed result: 1463264538 (3600s = one hour ahead).

For lulz, use another time zone:

    $ TZ=Asia/Novosibirsk git log --pretty=format:%ad --date=format:%s -1

Expected result: 1463260938 (unixtime is always UTC and should not
depend on TZ).
Observed result: 1463246538 (-14400s = 4 hours behind).

Not even specifying the UTC time zone helps:

    $ TZ=UTC git log --pretty=format:%ad --date=format:%s -1

Expected result: still 1463260938.
Observed result: 1463268138 (7200s = 2 hours ahead).