Git Tip of the Day - Committer vs Author

So one thing that has always kinda puzzled me in Git is that when I'd amend a commit, then do a git log the timestamp for that commit seemed unchanged even though I've effectively "rewritten" that commit. I dug into this a bit today and learned about the distinction between Author and Committer which proved insightful around this.

The distinction is well summarized in the Git docs:

You may be wondering what the difference is between author and committer. The author is the person who originally wrote the work, whereas the committer is the person who last applied the work.

So when you amend a commit you're updating the committer aspects of that commit and the author aspects remain unchanged. You can see both in Git log by using the --format=fuller argument:

$ git log --format=fuller
commit 9324ea7390b5c411c5cc050cf80965ce7425887a (HEAD -> foobar)
Author:     Adam Parkin <obfuscated@gmail.com>
AuthorDate: Fri Aug 6 11:37:15 2021 -0700
Commit:     Adam Parkin <obfuscated@gmail.com>
CommitDate: Fri Aug 6 11:37:15 2021 -0700

    Test commit

You can see in this that the commit in question was originally authored on August 6th, 2021 at 11:37AM (PST). Since this was the initial commit, the CommitDate is the same. But if we amend that commit & check again we'll see that the CommitDate is updated:

$ git commit --amend
[foobar 857e2a5] Test commit
 Date: Fri Aug 6 11:37:15 2021 -0700
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 content/gtotd-commit-vs-author.md

$ git log --format=fuller
commit 857e2a58ac0fe3c295ab80efe4cf21f42986fcfb (HEAD -> foobar)
Author:     Adam Parkin <obfuscated@gmail.com>
AuthorDate: Fri Aug 6 11:37:15 2021 -0700
Commit:     Adam Parkin <obfuscated@gmail.com>
CommitDate: Fri Aug 6 11:38:34 2021 -0700

    Test commit

In this case the amend was also done by me, so the author & committer are the same.

If you want to update the AuthorDate you can do so with the --date option to git commit:

$ git commit --amend --date 'now'
[foobar d45d5f9] Test commit
 Date: Fri Aug 6 11:40:34 2021 -0700
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 content/gtotd-commit-vs-author.md

$ git log --format=fuller
commit d45d5f9ea0579b6ea6fb4503a50abbac92a348a4 (HEAD -> foobar)
Author:     Adam Parkin <obfuscated@gmail.com>
AuthorDate: Fri Aug 6 11:40:34 2021 -0700
Commit:     Adam Parkin <obfuscated@gmail.com>
CommitDate: Fri Aug 6 11:40:34 2021 -0700

    Test commit

Here we can see that the use of --date set both AuthorDate and CommitDate to the same value (now).