Add "MODIFIED" (and "HASH") properties to Org-mode entries on save

org-mode-add-modified-property.el

;; Org-mode: Custom property: MODIFIED

;; Adapted from: https://emacs.stackexchange.com/a/39376/31079
(defun yant/getentryhash ()
  "Get the hash sum of the text in current entry.
Except :HASH: and :MODIFIED: property texts."
  (save-excursion
    (let* ((beg (progn (org-back-to-heading :invisible-ok) (point)))
           (end (progn
                  (forward-char)
                  (if (not (re-search-forward "^\*+ " (point-max) t))
                      (point-max)
                    (match-beginning 0))))
           (full-str (buffer-substring-no-properties beg end))
           (str-nohash (if (string-match "^ *:HASH:.+\n" full-str)
                           (replace-match "" nil nil full-str)
                         full-str))
           (str-nohash-nomod (if (string-match
                                  "^ *:MODIFIED:.+\n" str-nohash)
                                 (replace-match "" nil nil str-nohash)
                               str-nohash))
           (str-nohash-nomod-nopropbeg (if (string-match
                                            "^ *:PROPERTIES:\n"
                                            str-nohash-nomod)
                                           (replace-match
                                            "" nil nil
                                            str-nohash-nomod)
                                         str-nohash-nomod))
           (str-nohash-nomod-nopropbeg-end
            (if (string-match "^ *:END:\n" str-nohash-nomod-nopropbeg)
                (replace-match "" nil nil str-nohash-nomod-nopropbeg)
              str-nohash-nomod-nopropbeg))
           ;; Do not change hash just because whitespace was trimmed
           ;; from line-endings or extra newlines removed or added to
           ;; the end of an entry.
           (final-str
            (concat
             (string-trim-right
              (replace-regexp-in-string
               "[ \t]+[\n\r]" "\n"
               str-nohash-nomod-nopropbeg-end))
             "\n")))
      (secure-hash 'md5 final-str))))

(defun yant/update-modification-time ()
  "Set the :MODIFIED: property of the current entry.
Set to NOW and update :HASH: property."
  (org-set-property "HASH" (format "%s" (yant/getentryhash)))
  (org-set-property "MODIFIED"
                    (format-time-string
                     (org-time-stamp-format :with-hm :inactive)))
  (setq my/updated-org-entries-count
        (1+ my/updated-org-entries-count)))

(defun yant/skip-nonmodified ()
  "Skip org entries, which were not modified."
  (let ((next-headline (save-excursion (or (outline-next-heading)
                                           (point-max)))))
    (if (string= (org-entry-get (point) "HASH" nil)
                 (format "%s" (yant/getentryhash)))
        next-headline
      nil)))

(defvar my/updated-org-entries-count)
(defun my/org-update-modified-prop ()
  "Update the MODIFIED property if major-mode is `org-mode'."
  (setq-local my/updated-org-entries-count 0)
  (when (and (eq major-mode 'org-mode)
             (equal default-directory (expand-file-name "~/org/")))
    (org-map-entries #'yant/update-modification-time
                     nil
                     'file
                     #'yant/skip-nonmodified)
    (when (> my/updated-org-entries-count 0)
      (message "Saved %s and updated %s entries!"
               buffer-file-name
               my/updated-org-entries-count))))

(add-hook 'before-save-hook 'my/org-update-modified-prop)