git rebase without changing commit timestamps

git timestamp git-rebase

56,121

Solution 1

A crucial question of Von C helped me understand what is going on: when your rebase, the committer's timestamp changes, but not the author's timestamp, which suddenly all makes sense. So my question was actually not precise enough.

The answer is that rebase actually doesn't change the author's timestamps (you don't need to do anything for that), which suits me perfectly.

Solution 2

Update June 2014: David Fraser mentions in the comments a solution also detailed in "Change timestamps while rebasing git branch", using the option --committer-date-is-author-date (introduced initially in Jan. 2009 in commit 3f01ad6

Note that the --committer-date-is-author-date option seems to leave the author timestamp, and set the committer timestamp to be the same as the original author timestamp, which is what the OP Olivier Verdier wanted.

I found the last commit with the correct date and did:
git rebase --committer-date-is-author-date SHA

See git am:

--committer-date-is-author-date
By default the command records the date from the e-mail message as the commit author date, and uses the time of commit creation as the committer date.
This allows the user to lie about the committer date by using the same value as the author date.

Note: with Git 2.29 (Q4 2020), git rebase --committer-date-is-author-date or --ignore-date will also work with:

interactive rebase (rebase -i/rebase --interactive)
for the root commit (git rebase --root)

See "Change timestamps while rebasing git branch".

(Original answer, June 2012)

You could try, for a ~~non-interactive~~ rebase
(see just above: with Git 2.29, Q4 2020, that will work with an interactive rebase as well)

git rebase --ignore-date

(from this SO answer)

This is passed to git am, which mentions:

 --ignore-date

By default the command records the date from the e-mail message as the commit author date, and uses the time of commit creation as the committer date.
This allows the user to lie about the author date by using the same value as the committer date.

For git rebase, this option is "Incompatible with the --interactive option."

Since you can change at will the timestamp of old commit date (with git filter-branch), I suppose you can organize your Git history with whatever commit date order you want/need, even set it to the future!.

As Olivier mentions in his question, the author date is never changed by a rebase;
From the Pro Git Book:

The author is the person who originally wrote the work,

whereas the committer is the person who last applied the work.

So, if you send in a patch to a project and one of the core members applies the patch, both of you get credit.

To be extra clear, in this instance, as Olivier comments:

the --ignore-date does the opposite of what I was trying to achieve!
Namely, it erases the author's timestamp and replace them with the commits timestamps!
So the right answer to my question is:
Do not do anything, since git rebase does actually not change authors' timestamps by default.

As DylanYoung adds in the comments, using "How to identify conflicting commits by hash during git rebase?":

Using the SEQUENCE_EDITOR variable and rebase interactive, you would just loop over the current todo list and add a command setting the GIT_COMMITER_DATE to the date of the original commit before each commit in the todo.

It's a bit less fiddly because you have the list of original commits to start with (you don't have to hack into git internals to find it), but a bit more work because you have to handle the entire list at once. –

Once you're able to identify the original commit you can do something like:
git rebase -x 'GIT_COMMITTER_DATE="git show -s --format=%ci ``get_current_commit``" git commit --amend --no-edit

Solution 3

If you've already screwed up the commit dates (perhaps with a rebase) and want to reset them to their corresponding author dates, you can run:

git filter-branch --env-filter 'GIT_COMMITTER_DATE=$GIT_AUTHOR_DATE; export GIT_COMMITTER_DATE'

Solution 4

By default, git rebase will set the committer's timestamp to the time when the new commit is created, but keep the author's timestamp intact. Most of time, this is the desired behavior, but at some scenarios, we dot not wish to change the commiter's timestamp either. How can we accomplish that? Well, here is the trick I usually do.

First, make sure each of the commits you are about to rebase has a unique commit message and author timestamp (This is where is trick needs improvements, currently it suits my needs though).

Before the rebase, record the committer's timestamp, author's timestamp and commit message of all the commits which will be rebased to a file.

#NOTE: BASE is the commit where your rebase begins
git log --pretty='%ct %at %s' BASE..HEAD > hashlog

Then, let the actual rebase take place.

Finally, we replace the current committer's timestamp with the one recorded in the file if the commit message is the same by using git filter-branch.

 git filter-branch --env-filter '__date=$(__log=$(git log -1 --pretty="%at %s" $GIT_COMMIT); grep -m 1 "$__log" ../../hashlog | cut -d" " -f1); test -n "$__date" && export GIT_COMMITTER_DATE=$__date || cat'

If something goes wrong, just checkout git reflog or all the refs/original/ refs.

Furthormore, you can do the similar thing to the author's timestamp.

For example, if the author's timestamp of some commits are out of order, and without rearrange these commits, we just want the author's timestamp to show in order, then the following commands will help.

git log --pretty='%at %s' COMMIT1..COMMIT2 > hashlog
join -1 1 -2 1 <(cat hashlog | cut -f 1 | sort -nr | awk '{ print NR" "$1 }') <(cat hashlog | awk '{ print NR" "$0 }') | cut -d" " -f2,4- > hashlog_
mv hashlog_ hashlog
git filter-branch --env-filter '__date=$(__log=$(git log -1 --pretty="%s" $GIT_COMMIT); grep -m 1 "$__log" ../../hashlog | cut -d" " -f1); test -n "$__date" && export GIT_AUTHOR_DATE=$__date || cat'

Solution 5

post-rewrite hook

This hook works for all of git rebase, git pull --rebase and git commit --amend.

.git/hooks/post-rewrite

set -eu
echo post-rewrite
if [ ! "${CIROSANTILLI_GITHOOKS_DISABLE:-0}" = 1 ]; then
  declare -a olds
  declare -A oldnew
  while IFS= read -r line; do
    echo "$line"
    old="$(echo "$line" | cut -d ' ' -f1)"
    new="$(echo "$line" | cut -d ' ' -f2)"
    oldnew[$old]="$new"
    olds+=("$old")
    news+=("$new")
  done
  git reset --hard "${news[0]}~"
  for old in "${olds[@]}"; do
    new="${oldnew[$old]}"
    git cherry-pick "$new" &>/dev/null
    olddate="$(git log --format='%cd' -n 1 "$old")"
    CIROSANTILLI_GITHOOKS_DISABLE=1 \
      GIT_COMMITTER_DATE="$olddate" \
      git commit \
      --amend \
      --no-edit \
      --no-verify \
      &>/dev/null \
    ;
  done
  echo
fi

GitHub upstream.

Don't forget to:

chmod +x .git/hooks/post-rewrite

This is a good way to do --committer-date-is-author-date by default on chosen repos, before someone finally patches a config to set it by default.

And it also works with --committer-date-is-author-date which does not seem to be exposed on git pull --rebase.

Olivier Verdier

Updated on December 03, 2021

Comments

Olivier Verdier over 2 years
Would it make sense to perform git rebase while preserving the commit timestamps?

I believe a consequence would be that the new branch will not necessarily have commit dates chronologically. Is that theoretically possible at all? (e.g. using plumbing commands; just curious here)

If it is theoretically possible, then is it possible in practice with rebase, not to change the timestamps?

For example, assume I have the following tree:
```
master <jun 2010>
  |
  :
  :
  :     oldbranch <feb 1984>
  :     /
oldcommit <jan 1984>
```
Now, if I rebase oldbranch on master, the date of the commit changes from feb 1984 to jun 2010. Is it possible to change that behaviour so that the commit timestamp is not changed? In the end I would thus obtain:
```
      oldbranch <feb 1984>
      /
 master <jun 2010>
    |
    :
```
Would that make sense at all? Is it even allowed in git to have a history where an old commit has a more recent commit as a parent?
- pfalcon almost 12 years
  
  It's funky that the answer to the question is indeed "you don't need to do anything - that's how it works be default". But now suppose you want commit to be sorted in proper date order while doing rebase (which is pretty natural scenario if you think about it). Now, I wasn't able to find how to achieve that, and posted my q as stackoverflow.com/questions/12270357/really-flatten-a-git-me‌rge
- VonC about 10 years
  
  David mentions another option to reset committer date: git rebase --committer-date-is-author-date SHA. See my edited answer below
- axiac about 9 years
  
  I just wrote an extensive answer on a similar question whose author tried the answers explained here and couldn't apply them in a satisfactory manner.
Olivier Verdier about 14 years

Interesting about the arbitrary dates to commit. However, git rebase --ignore-date does not work. It changes the dates of the rebased commits.
VonC about 14 years

@Olivier: strange: did you made a non-interactive rebase? And between the Author Date and the Committer Date, are we sure to monitor the "right" date?
Olivier Verdier about 14 years

Thanks VonC, the difference between author and committer timestamp, that's what makes it all suddenly clear. I wrote the answer to my question in my post, but feel free to adapt your answer to reflect that.
VonC about 14 years

@Olivier: good point. I have completed my answer to reflect the difference between the two dates.
Olivier Verdier about 14 years

to be more precise: the --ignore-date does the opposite of what I was trying to achieve! Namely, it erases the author's timestamp and replace them with the commits timestamps! So the right answer to my question is: do not do anything, since git rebase does actually not change authors' timestamps by default.
Markus N. over 10 years

I just tried this, but no effect. I got the folowing output: WARNING: Ref 'refs/heads/master' is unchanged. I am using git version 1.7.9.5 on Linux (64 bit)
speakman about 10 years

I'd like to add another approach if you've already screwed up but don't want to iterate through the whole history: git rebase --committer-date-is-author-date <base_branch> This way, git will reset the commit date only for the commits applied upon <base_branch> (which is probably the same branch name you used when you screwed up).
David Fraser about 10 years

Note that the --committer-date-is-author-date option seems to leave the author timestamp, and set the committer timestamp to be the same as the original author timestamp, which is what Olivier wanted...
VonC about 10 years

@DavidFraser good point. I have included your comment in the answer for more visibility.
1615903 over 8 years

+1 - I have a git alias in place (coderwall.com/p/euwpig/a-better-git-log) that apparently uses the committer's timestamp, which was confusing me. Gitk and git log both show the author's timestamp.
Theodore R. Smith about 8 years

The accepted answer didn't work in 2016, but @speakman's answer did!
endolith almost 8 years

Running git rebase --committer-date-is-author-date SHA_of_root_commit on all of my branches zipped them all back together again! Yay!
Amedee Van Gasse over 7 years

@speakman's answer didn't work in October 2016, but Andy's did!
vaindil over 7 years

This does not work on Windows. I was able to get it to work using Windows Bash.
Zack Morris almost 7 years

stackoverflow.com/a/11179245/539149 worked as of 2017/07/31, stackoverflow.com/questions/2973996/… did not. It's too bad that git so often chooses non-human-oriented defaults due to its unix roots, but thankfully is powerful enough to undo them when needed.
audun over 6 years

This is a great trick! It allowed me to only rewrite 75 commits instead of 1100+ from using the other answers.
bsb over 6 years

Tip: check status with: git log --pretty=fuller | egrep '^commit|Date'
David DeMar over 6 years

This is fantastic! Is there a way to modify the script to also preserve the original committer as well?
weynhamz over 6 years

@DavidDeMar should be, the same, just change the git log --pretty to record the original email, and modify the script accordingly.
VonC over 5 years

Very interesting. +1
Utku over 3 years

@VonC In short, there is no way to preserve the committer date with git-rebase right? The best you can do is to set the committer date equal to the author date.
VonC over 3 years

@Utku Yes, that is the idea.
DylanYoung about 3 years

Sure you can. Use git rebase -x or git rebase -i with the SEQUENCE_EDITOR environment variable set to a little script, but there are lots of edge cases you'll have to handle I suspect.
VonC about 3 years

@DylanYoung do you have an example of such a script?
DylanYoung about 3 years

Not off the top or I'd add an answer, but I believe this is the key: https://stackoverflow.com/questions/2118364/how-to-identify-‌conflicting-commits-‌by-hash-during-git-r‌ebase. Once you're able to identify the original commit you can do something like: git rebase -x 'GIT_COMMITTER_DATE="git show -s --format=%ci ``get_current_commit``" git commit --amend --no-edit'
DylanYoung about 3 years

Forgive the formatting, not sure how to do nested backticks in SO.
DylanYoung about 3 years

Using the SEQUENCE_EDITOR variable and rebase interactive, you would just loop over the current todo list and add a command setting the GIT_COMMITER_DATE to the date of the original commit before each commit in the todo. It's a bit less fiddly because you have the list of original commits to start with (you don't have to hack into git internals to find it), but a bit more work because you have to handle the entire list at once.
VonC about 3 years

@DylanYoung Interesting. I have tried and included your comment in the answer. Please edit it if I have misconstrued your contribution.