It's not; the git format defines it as a positive integer, see git help commit:
DATE FORMATS
The GIT_AUTHOR_DATE and GIT_COMMITTER_DATE environment variables support the following date formats:
Git internal format
It is <unix-timestamp> <time-zone-offset>, where <unix-timestamp> is the number of seconds since the UNIX epoch.
Changing a commit's timestamp is as simple as: $ git commit --amend --date='1970-01-01T00:00:00' --reset-author
[main 6e1d001] test
Date: Thu Jan 1 00:00:00 1970 +0000
1 file changed, 1 insertion(+)
But dates before 1970 really don't work (in some cases it gives "fatal: invalid date format"): TZ=UTC git commit --amend --date='1969-12-31T23:59:59Z' --reset-author
[main 47e54f0] test
Date: Mon Dec 31 23:59:59 2012 +0000
1 file changed, 1 insertion(+)
