The answers to Edit an incorrect commit message in Git 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?
use git filter-branch with an env filter that sets GIT_AUTHOR_DATE and GIT_COMMITTER_DATE for the specific hash of the commit you're looking to fix.
This will invalidate that and all future hashes.
Edited for example
If you wanted to change the dates of commit
119f9ecf58069b265ab22f1f97d2b648faf932e0, you could do so with something like this:
git filter-branch --env-filter \ 'if [ $GIT_COMMIT = 119f9ecf58069b265ab22f1f97d2b648faf932e0 ] then export GIT_AUTHOR_DATE="Fri Jan 2 21:38:53 2009 -0800" export GIT_COMMITTER_DATE="Sat May 19 01:01:01 2007 -0700" fi'