Making Git Commits in the Past

Recently on a side project I’ve been working on I decided that I wanted to create a repository… 2 weeks later. This was because initially I felt I didn’t have much code to begin with, no point keeping a version control of code that hasn’t been written yet.. right?

By the time I decided to initialize a git for it, I decided I wanted to keep track of when I started, so I can have a gauge of the duration it took for me to develop the program, at least on whatever free off work hours I could find. Searching online didn’t lead me to the answers right away, so here’s how to create a commit back in time.

Scenario: You suddenly want to change the timestamp of a commit in git to a time in the past. You don’t want to resort to changing your system’s clock, and you don’t really want to (and well, can’t) use a time machine to go back in time to make a commit.

The gist (haha) of it is to manually change the values for GIT_COMMITTER_DATE and also GIT_AUTHOR_DATE.

If you have an empty repository.

If you have a brand new repository right after git init or cloning down from GitHub or GitLab or any other provider, open a new terminal and do:

$ cd path/to/git/project`
$ export GIT_COMMITTER_DATE="YYYY-MM-DD HH:MM:SS"
$ export GIT_AUTHOR_DATE="YYYY-MM-DD HH:MM:SS"
$ git commit -am 'Commit Message Here'

The format for dates can be done in the above fashion, or another format such as “Sat Aug 2 15:00”, RFC 2822 and ISO 8601 date formats are valid. More information on the date format here

The reason why we need to do both committer and author date is because they mean different things in Git, and also GitHub/GitLab/other service providers. Usually they are stored as the same values, since if you do a commit without specifying a custom date for any of them the values default to ‘now’, which is the time you made the commit. The different, as git-scm explains is that the author is the person who originally wrote the work while the committer is the person who last applied the work.

A word of caution though, if you decide to continue making more commits after making that commit in-the-past. Close your terminal after making the in-the-past commit or use another one! I made the mistake of making more commits in that terminal and they all used the exported value of the author and committer date, messing up my entire commit history!

What about the --date switch?

Good question, I initially did my back to the past commit using that switch, but upon pushing it to GitLab I realized that the date shown was not in the past, but rather at the time I made the git commit command. Apparently GitLab (not sure about GitHub, haven’t tried) uses the commit date and not the author date. Whereas the --date switch only allows you to specify the author date (meaning only GIT_AUTHOR_DATE is modified, not GIT_COMMITTER_DATE). Bummer.

Bonus tip: If you’re just going to change the timestamps of the previous commit, go ahead and use the --amend flag:

$ git commit --amend --date="YYYY-MM-DD HH:MM:SS"

Big credits to this post by Alex Peattie on highlighting the differences on dates, check it out, the post goes way more in-depth on working with git dates, as well as approximate dates and even a script to automate some processes! (Wonder if there’s one for real life romantic dates too..)

Wait, what about existing repo’s with history?

Well, assuming you have a commit history like so (meaning it isn’t a fresh new repo) and you want to change the initial commit:

5ed237af	Added test cases (HEAD)
234cd89e	Implementation of feature X 		
144ab94e	Initial Commit 

What you want to do is first do a rebase with whichever commit you want, an interactive one is usually suggested, from what I’ve found online. And then export those GIT_AUTHOR_DATE and GIT_COMMITTER_DATE values like I highlighted above, commit, close the terminal and then continue your rebase in another one to erase the exported value. (Or you could continue exporting other values to make it go back in time)

$ git rebase -i HEAD~2

Change ‘pick’ to ‘edit’ for the commit you want, then save. (use the no edit flag if you don’t need to change to message)

$ git commit --all --amend --no-edit

Close your terminal or reset the exported values (anyone know how to do this painlessly please comment!), then finally do (in a new terminal):

$ git rebase --continue

I initially tried this route but had issues (I had an empty commit before this, just a placeholder file and git didn’t display that somehow, it was the commit I wanted to change the date for), and forgot to close my terminal so I ended up with multiple commits at the same time. Since I was just starting out it was just easier for me to delete my git and start over.

Read more on Stack Overflow on how to modify a specific commit and making a commit in the past.

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