GitHub Pages用のmarkdownファイルに"update:"という行が現れたら、markdownファイルの保存時にそれらのうちの最初の行に最終更新日時を追加するためのEmacs Lispのプログラム。

insert-timestamp-for-githubpages-for-markdown.el

;; See https://pandanote.info/?p=6871 for details.
(defun insert-timestamp-for-githubpages ()
  "Insert time stamp into the line starting from \"update: \"."
  (interactive)
  (goto-char 0)
  (if (re-search-forward "^update:.*$" nil t)
  (replace-match (concat "update: " (current-time-string) " " (format-time-string "%z"))))
  )

(defun insert-timestamp-for-githubpages-for-markdown ()
  "Check the major mode to invoke insert-timestamp-for-githubpages."
  (when (eq major-mode 'markdown-mode)
	(insert-timestamp-for-githubpages))
  )

(add-hook 'before-save-hook #'insert-timestamp-for-githubpages-for-markdown)