Truncate Git repo history, back-date initial commit, push to new repo

README.md

1. Check out the branch you want to truncate.
2. Get the SHA1 hash of the commit in that branch you want to start history.
3. truncate.sh SHA1 branchname like truncate.sh 01cea49c9cbefd696bbd54ee41b47cbf1b6d1bd5 master
4. Get the SHA1 hash of the new base commit.
5. Get the date you want that commit to have in the format like Sat, 14 Dec 2013 12:40:00 -0800
6. fixdate.sh SHA1 "date" like fixdate.sh c776e91dd573053ec09ecb1b0737909f9aacd0c9 "Sat, 14 Dec 2013 12:40:00 -0800"
7. Add a remote to the new repo.
8. Push the updated branch to the remote.

fixdate.sh

#!/bin/bash
git filter-branch --env-filter \
"if test \$GIT_COMMIT = '$1'
then
    export GIT_AUTHOR_DATE='$2'
    export GIT_COMMITTER_DATE='$2'
fi" && rm -fr "$(git rev-parse --git-dir)/refs/original/"

truncate.sh

#!/bin/bash
git checkout --orphan temp $1
git commit -m "Truncated history."
git rebase --onto temp $1 $2
git branch -D temp