Git/History/Rewriting

via https://www.reddit.com/r/git/comments/2pjgsx/git_how_do_i_modify_timestamps_on_commit_objects/

How to modify entire commit history

Suppose you want to modify your entire git commit history. To do that, you can use the git filter-branch command to run a command/script on every commit that can be reached from HEAD (i.e., starting from HEAD and traversing backwards to root).

git filter-branch --commit-filter CMD 

CMD is a command that is passed the same arguments as git commit-tree.

It should do what git commit-tree does.

(Which is...?)

Here's an example (executable) shell script called /tmp/rewrite.sh:

#!/bin/sh
export GIT_AUTHOR_DATE=1234567890+0000
export GIT_COMMITTER_DATE=1234567890+0000
git commit-tree "$@"

Now, when you rewrite the history, the entire history will appear to have been committed at the same timestamp, 1234567890:

Before:

$ git log
commit 21a52d0c3a27faa1f8a0c3dc894ffb3f529b0d65
Author: Your Name
Date:   Tue Dec 16 23:50:51 2014 -0500

    second commit

commit 2e7a613b24c55ba383440a88dc9b5b55cddca5ad
Author: Your Name
Date:   Tue Dec 16 23:50:38 2014 -0500

    first commit

Apply git filter-branch:

$ git filter-branch --commit-filter '/tmp/rewrite.sh "$@"'
Rewrite 21a52d0c3a27faa1f8a0c3dc894ffb3f529b0d65 (2/2)
Ref 'refs/heads/master' was rewritten

After:

$ git log
commit 15bf1d225ffb17103e0476687dcd6e0a5ce683c8
Author: Your Name
Date:   Fri Feb 13 23:31:30 2009 +0000

    second commit

commit b864f505b2a923b69f9da8dc7cb1c19d45dad92f
Author: Your Name
Date:   Fri Feb 13 23:31:30 2009 +0000

    first commit

Original refs are still stored in .git/refs/original:

$ cat .git/refs/original/refs/heads/master
21a52d0c3a27faa1f8a0c3dc894ffb3f529b0d65

The filter command is passed the same arguments as git commit-tree.

The first argument will be the tree of the commit to rewrite.

You could write a more complex script which views the contents of that tree and sets the date appropriately.