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

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"

Afterwards you continue your interactive rebase.

UPDATE (in response to the comment of studgeek): to change the commit date instead of the author date:

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

The lines above set an environment variable GIT_COMMITTER_DATE which is used in amend commit.

Everything is tested in Git Bash.

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.

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'

A better way to handle all of these suggestions in one command is

LC_ALL=C GIT_COMMITTER_DATE="$(date)" git commit --amend --no-edit --date "$(date)"

This will set the last commit's commit and author date to "right now."