Change the timestamp of previous Git commits

git-timemachine.sh

# Git has two different types of timestamp associated with a commit. 
# Although both may typically hold the same value they are used in subtly different ways. 
# The author (GIT_AUTHOR_DATE) is the user who originally created the work (i.e. a patch), 
# where as the committer (GIT_COMMITTER_DATE) is the user who last applied the 
# work (i.e. applied patch or rebase).

git filter-branch --env-filter \
"if test \$GIT_COMMIT = 'e6dbcffca68e4b51887ef660e2389052193ba4f4'
then
    export GIT_AUTHOR_DATE='Sat, 14 Dec 2013 12:40:00 +0000'
    export GIT_COMMITTER_DATE='Sat, 14 Dec 2013 12:40:00 +0000'
fi" && rm -fr "$(git rev-parse --git-dir)/refs/original/"