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

Question

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 t

Answer 1

Here is a convenient alias that changes both commit and author times of the last commit to a time accepted by date --date:

[alias]
    cd = "!d=\"$(date -d \"$1\")\" && shift && GIT_COMMITTER_DATE=\"$d\" \
            git commit --amend --date \"$d\""

Usage: git cd

Examples:

git cd now  # update the last commit time to current time
git cd '1 hour ago'  # set time to 1 hour ago

Edit: Here is a more-automated version which checks that the index is clean (no uncommitted changes) and reuses the last commit message, or fails otherwise (fool-proof):

[alias]
    cd = "!d=\"$(date -d \"$1\")\" && shift && \
        git diff-index --cached --quiet HEAD --ignore-submodules -- && \
        GIT_COMMITTER_DATE=\"$d\" git commit --amend -C HEAD --date \"$d\"" \
        || echo >&2 "error: date change failed: index not clean!"