git fix-dates: Fix GitHub Commit Dates After a Rebase

#!/usr/bin/env bash
# Make every commit's committer date equal its author date, then force-push,
# so github.com shows the same dates as GitHub Desktop.
#
# Usage: git fix-dates [-n] [-y] [--no-push] [remote]    (see -h)
set -euo pipefail

usage() {
  cat <<'EOF'
Usage: git fix-dates [options] [remote]

Rewrite the current branch so every commit's committer date equals its author
date, then force-push it. <remote> defaults to the branch's upstream, else origin.

Options:
  -n, --dry-run   Report mismatched commits and exit; change nothing
  -y, --yes       Skip the confirmation prompt (required when not on a TTY)
      --no-push   Rewrite locally but don't push
  -h, --help      Show this help
EOF
}

die() {
  printf 'git fix-dates: %s\n' "$*" >&2
  exit 1
}

# Ask "<question>? [y/N]" unless --yes was given; anything but yes aborts.
confirm() {
  $assume_yes && return
  [ -t 0 ] || die "refusing to continue non-interactively without --yes"
  read -r -p "$1? [y/N] " reply
  case $reply in [yY] | [yY][eE][sS]) ;; *) die "aborted" ;; esac
  confirmed=true
}

dry_run=false
assume_yes=false
confirmed=false
push=true
remote=

while [ $# -gt 0 ]; do
  case $1 in
    -n | --dry-run) dry_run=true ;;
    -y | --yes) assume_yes=true ;;
    --no-push) push=false ;;
    -h | --help) usage; exit 0 ;;
    -*) die "unknown option '$1' (see -h)" ;;
    *)
      [ -z "$remote" ] || die "unexpected argument '$1' (see -h)"
      remote=$1
      ;;
  esac
  shift
done

# --- Preflight: refuse anything that would make a rewrite unsafe ------------

git rev-parse --is-inside-work-tree >/dev/null 2>&1 || die "not inside a Git work tree"
branch=$(git symbolic-ref --quiet --short HEAD) || die "HEAD is detached; check out a branch first"

git_dir=$(git rev-parse --git-dir)
for marker in rebase-merge rebase-apply MERGE_HEAD CHERRY_PICK_HEAD REVERT_HEAD BISECT_LOG; do
  [ ! -e "$git_dir/$marker" ] || die "a rebase, merge, cherry-pick, revert or bisect is in progress; finish it first"
done

remote=${remote:-$(git config --get "branch.$branch.remote" || echo origin)}
if $push && ! $dry_run; then
  git remote get-url "$remote" >/dev/null 2>&1 || die "no such remote '$remote'"
fi

# --- Find mismatched commits ------------------------------------------------

# Topological order guarantees `first` has no mismatched ancestor, so
# rewriting `first^..branch` covers every mismatch on the branch.
read -r count first < <(
  git log --topo-order --reverse --format='%H %at %ct' "$branch" |
    awk '$2 != $3 { n++; if (!f) f = $1 } END { print n + 0, f }'
)

if [ "$count" -eq 0 ]; then
  echo "All commit dates on '$branch' already match."
else
  echo "$count commit(s) on '$branch' have mismatched dates (oldest: $(git log -1 --format='%h %s' "$first"))."
  $dry_run && exit 0

  # filter-branch checks out the result, so uncommitted work would be at risk.
  git diff-files --quiet --ignore-submodules && git diff-index --cached --quiet --ignore-submodules HEAD ||
    die "working tree has uncommitted changes; commit or stash them first"

  action="Rewrite history of '$branch'"
  $push && action="$action and force-push to '$remote'"
  confirm "$action"

  old_head=$(git rev-parse HEAD)
  range=$branch
  parent=$(git rev-parse --quiet --verify "$first^") && range="$parent..$branch"

  # Unlike `git rebase`, this copies trees verbatim, so merges survive,
  # nothing can conflict, and the original committer name/email are kept.
  # The date is set only for commit-tree: an exported one (--env-filter) would
  # also backdate the branch's reflog entry and trip --force-if-includes.
  FILTER_BRANCH_SQUELCH_WARNING=1 git filter-branch --force \
    --commit-filter 'GIT_COMMITTER_DATE=$GIT_AUTHOR_DATE git commit-tree "$@"' \
    -- "$range" >/dev/null
  git update-ref -d "refs/original/refs/heads/$branch"

  echo "Rewritten. Undo with: git reset --hard $old_head"
fi

$dry_run && exit 0
$push || exit 0

# --- Push -------------------------------------------------------------------

# Lease against the remote-tracking ref without fetching first — a fetch would
# refresh it and let the force-push silently discard someone else's commits.
# --force-if-includes also refuses if that ref holds commits you never had
# locally (fetched but not pulled). It only works without an explicit <expect>.
# Nothing rewritten (e.g. after --no-push) means nothing confirmed yet, so ask.
tracking="refs/remotes/$remote/$branch"
if lease=$(git rev-parse --quiet --verify "$tracking"); then
  if [ "$(git rev-parse HEAD)" = "$lease" ]; then
    echo "'$remote/$branch' is already up to date."
  else
    $confirmed || confirm "Force-push '$branch' to '$remote'"
    git push --force-with-lease="refs/heads/$branch" --force-if-includes \
      "$remote" "refs/heads/$branch:refs/heads/$branch" ||
      die "push rejected; '$remote/$branch' has commits your branch doesn't — fetch, integrate them, then retry"
  fi
else
  # Never seen this branch on the remote: a plain push fails if it exists there.
  $confirmed || confirm "Push '$branch' to '$remote'"
  git push --set-upstream "$remote" "refs/heads/$branch:refs/heads/$branch"
fi

mkdir -p ~/.local/bin
# Save the script above as ~/.local/bin/git-fix-dates, then:
chmod +x ~/.local/bin/git-fix-dates

# Make sure ~/.local/bin is on your PATH (add this to ~/.bashrc or ~/.zshrc)
export PATH="$HOME/.local/bin:$PATH"

git fix-dates -h

git fix-dates -n            # Dry run: report mismatches, change nothing
git fix-dates               # Rewrite (after a y/N prompt) and force-push to the upstream remote
git fix-dates -y            # Skip the prompt — for scripts or when you're sure
git fix-dates --no-push     # Rewrite locally only; push later yourself
git fix-dates fork          # Push to a remote other than the branch's upstream

$ git fix-dates
3 commit(s) on 'main' have mismatched dates (oldest: e467a53 Add search index).
Rewrite history of 'main' and force-push to 'origin'? [y/N] y
Rewritten. Undo with: git reset --hard fdf480a7e3066897d9ce5004dd3012f621ac317a
To github.com:you/repo.git
 + fdf480a...671d9cf main -> main (forced update)

# Show both dates side by side to spot drift
git log --format='%h  author %ad  committer %cd  %s' --date=format:'%Y-%m-%d %H:%M'

# List only the commits whose dates differ
git log --format='%h %at %ct %s' | awk '$2 != $3'

# Fix dates while rebasing in the first place (linear history only)
git rebase --committer-date-is-author-date main

# Amend without moving the author date forward (default), or deliberately reset it to now
git commit --amend --no-edit
git commit --amend --no-edit --reset-author

# Recover if a rewrite went wrong — the reflog remembers every previous HEAD
git reflog -10
git reset --hard HEAD@{1}