[Bug] git-log prints wrong unixtime with --date=format:%s

Hello,

the command

   git log --pretty=format:%ad --date=format:%s

displays wrong unixtime values; apparently how much the printed value differs from the expected value depends on the system's time zone and whether daylight savings time is enabled or not.

Here is a reproducible recipe compiled by Yuri Khan who helped me localizing the problem and whom I CC'd:

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).

FWIW, personally I have not bound the TZ environment variable, my time zone is constantly that of Berlin, currently CEST.

Many thanks in advance,

Michael.