> git dates are unsigned timestamps

You are thinking of the in-memory representation of the (main) implementation, not the git format itself.

The on-disk/wire format is text representation, encoded in decimal. You can add a minus sign in front of it, git won't crash when parsing it. The implementation will, however, display the wrong date on git log, but it's a bug that git developers are working on.

For example (coincidentally, it is also a constitution):

    $ git init
    $ curl https://archive.softwareheritage.org/api/1/vault/revision/5e8f80061263d818ae3fc6c8d2086b5278f271ea/gitfast/raw/ | gunzip | git fast-import > /dev/null
    $ git show 5e8f80061263d818ae3fc6c8d2086b5278f271e | grep Date
    Date:   Thu Jan 1 00:00:00 1970 +0000
    $ git cat-file -p 5e8f80061263d818ae3fc6c8d2086b5278f271e | grep author
    author State Constitutional Conventions <> -5706313200 -0455