That's the who not the when or how

For the when:

GIT_COMMITTER_DATE="Tue Dec 8 12:33:03 2015 +0000" git commit --date="Tue Dec 8 12:33:03 2015 +0000"

That will change the commit and author dates.

Shouldn't the server log incoming pushes so you can find when the commit arrived, not just when the commit claims to have been made?

Maybe they don't, but this seems like something they ought to do.

You could have them do it, but it's just going deeper down the rabbit hole. The eventual question is "who/what do you trust?" - maybe it was the git server that got pwned?

A PGP-signed commit with a key generated on a smartcard (and never exposed) is a little better, but ... Someone pwned RSA before, and I'd be surprised if Gemalto and Yubico (just two examples) don't have some Three-Letter-Agency backdoor (and .. I'm sure those TLAs have equipment that can read modern smartcards).

Difference between CMS & VCS.