Changing the Timestamp of a Previous Git Commit
Git has two different types of timestamp associated with a commit.
Although both typically hold the same value, they are used in subtly different ways.
The author (GIT_AUTHOR_DATE) is the user who originally created the work (i.e. a patch), whereas the committer (GIT_COMMITTER_DATE) is the user who last applied the work (i.e. applied patch or rebase).
The author date is the one displayed when the log is accessed.
However, the commit date is used when applying the --since and --until filter options, which seems a little odd.
To avoid confusion, you can include the committer date within your log display by setting the --format option.
$ git log --format=fuller
Ch-ch-Changes…
A tendency I have picked up is to be very commit-happy when developing locally, and then I often find myself rebasing frequently before pushing to a remote. As a result, I am sometimes required to alter the timestamps of certain commits to make them more meaningful. I found that the simplest approach is to add a specified timestamp to the current commit. The author date is specified with the –date option, whereas the committer timestamp must be changed using an environment variable.
$ git commit --date="Sat, 14 Dec 2013 12:40:00 +0000" # only author
$ GIT_COMMITTER_DATE="`date -R`" git commit --date "`date -R`" # for both
If, on the other hand, we wish to amend the last author commit, we can execute the following similar command. I should warn you to be wary of running these commands on a branch with commits that have already been pushed to a remote, as Git’s hashing nature will alter the SHAs of all subsequent commits.
$ git commit --amend --date "`date -R`" # include '-C HEAD' to bypass commit message editing
Finally, to alter a previous commit by its SHA reference hash, you can run the following command (adapted from GitFaq).
$ git filter-branch --env-filter \
"if test \$GIT_COMMIT = 'e6dbcffca68e4b51887ef660e2389052193ba4f4'
then
export GIT_AUTHOR_DATE='Sat, 14 Dec 2013 12:40:00 +0000'
export GIT_COMMITTER_DATE='Sat, 14 Dec 2013 12:40:00 +0000'
fi" && rm -fr "$(git rev-parse --git-dir)/refs/original/"
Viewing the alterations
We can now review our alterations by checking the log again.
This is achieved by using the --since and --until options to filter the results.
$ git log --since="yesterday"
$ git log --since="1 month ago" --until="yesterday"