How can one change the timestamp of an old commit in Git?

The answers to How to modify existing, unpushed commits? describe a way to amend previous commit messages that haven't yet been pushed upstream. The new messages inherit the timestamps of the original commits. This seems logical, but is there a way to also re-set the times?

[machupicchubeta]

“git commit --amend --reset-author --no-edit”

[Nyoho]

時刻の歴史を改変するの

[kitokitoki]

コミット日付の変更

[yyamano]

git commit --amend --date="Wed Feb 16 14:00 2011 +0100"