git-reset-date-order

#!/bin/bash

set -euo pipefail

if [[ ${#} -lt 1 ]]; then
  echo "Usage: git-reset-date-order BRANCH_UPSTREAM

Resets the author and committer dates of each commit of a branch so they are in order.  The
reference date is the committer date of the HEAD commit, and the date gets decremented by 1 minute
for every parent commits.

Argument:
  BRANCH_UPSTREAM: the base commit of your current branch (ex: origin/master)"
  exit 1
fi

date=$(date --date "$(git log -1 --pretty='%cI' HEAD)" '+%s')
upstream=${1}

filter_script=$(
  for hash in $(git log --pretty=format:%H "${upstream}..HEAD"); do
    commit_date=$(date --date="@$date" -Iseconds)

    echo "
if [ \$GIT_COMMIT = ${hash} ];
then
    export GIT_AUTHOR_DATE=\"$commit_date\"
    export GIT_COMMITTER_DATE=\"$commit_date\";
fi;
"

    let "date = $date - 60"
  done
)

previous_head=$(git rev-parse HEAD)
FILTER_BRANCH_SQUELCH_WARNING=1 git filter-branch -f --env-filter "$filter_script" "$upstream..HEAD"
echo "Done. To cancel changes, run:
  git reset --hard $previous_head"