You can do an interactive rebase and choose
edit for the commit whose date you would like to alter. When the rebase process stops for amending the commit you type in for instance:
git commit --amend --date="Wed Feb 16 14:00 2011 +0100" --no-edit
--date=now will use the current time.
Afterward, you continue your interactive rebase.
To change the commit date instead of the author date:
GIT_COMMITTER_DATE="Wed Feb 16 14:00 2011 +0100" git commit --amend --no-edit
The lines above set an environment variable
GIT_COMMITTER_DATE which is used in amending commit.
Everything is tested in Git Bash.