$ git commit --date '1999-01-01' -m "foo" --allow-empty | grep Date
 Date: Fri Jan 1 12:59:34 1999 +0100
$ git commit --date '1973-03-03' -m "foo" --allow-empty | grep Date
 Date: Sat Mar 3 13:00:21 1973 +0100
$ git commit --date '1973-03-02' -m "foo" --allow-empty | grep Date
fatal: invalid date format: @99921632
$ git commit --date '1970-01-01' -m "foo" --allow-empty | grep Date
fatal: invalid date format: @43252
$ git commit --date '1969-12-31' -m "foo" --allow-empty | grep Date
 Date: Mon Dec 31 13:01:02 2012 +0100