pre-commit

#!/bin/bash
set -eEuo pipefail

# Method 1: a git pre-commit hook

date=$(date +%Y%m%d%H%M)

git diff-index --name-only --cached -z HEAD |
	( grep -z '\.el$' || true ) |
	while read -r -d '' fn
	do
		# Patch index
		blob_sha1=$(git show :"$fn" |
						sed "s/^;; Version:  .*/;; Version:  $date/" |
						git hash-object -w --stdin)
		printf '100644 blob %s\t%s\0' "$blob_sha1" "$fn" |
			git update-index -z --index-info

		# Patch working tree to match index
		sed -i "s/^;; Version:  .*/;; Version:  $date/" "$fn"
	done

update-package-version.el

;; Method 2: an Emacs before-save-hook

(defun d-mode-update-package-version ()
  "Update package version timestamp e.g. in d-mode.el."
  (interactive)
  (when (eq major-mode 'emacs-lisp-mode)
    (save-excursion
      (save-restriction
	(widen)
	(goto-char (point-min))
	(when (re-search-forward "^;; Version:  [0-9]\\{12\\}$")
	  (delete-char -12)
	  (insert (format-time-string "%Y%m%d%H%M" nil t)))))))

(add-hook 'before-save-hook 'd-mode-update-package-version)