.emacs.el

(defvar org-my-archive-expiry-days 1
  "The number of days after which a completed task should be auto-archived.
This can be 0 for immediate, or a floating point value.")

(defconst org-my-ts-regexp "[[<]\\([0-9]\\{4\\}-[0-9]\\{2\\}-[0-9]\\{2\\} [^]>\r\n]*?\\)[]>]"
  "Regular expression for fast inactive time stamp matching.")

(defun org-my-closing-time ()
  (let* ((state-regexp
          (concat "- State \"\\(?:" (regexp-opt org-done-keywords)
                  "\\)\"\\s-*\\[\\([^]\n]+\\)\\]"))
         (regexp (concat "\\(" state-regexp "\\|" org-my-ts-regexp "\\)"))
         (end (save-excursion
                (outline-next-heading)
                (point)))
         begin
         end-time)
    (goto-char (line-beginning-position))
    (while (re-search-forward regexp end t)
      (let ((moment (org-parse-time-string (match-string 1))))
        (if (or (not end-time)
                (time-less-p (apply #'encode-time end-time)
                             (apply #'encode-time moment)))
            (setq end-time moment))))
    (goto-char end)
    end-time))

(defun org-my-archive-done-tasks ()
  (interactive)
  (save-excursion
    (goto-char (point-min))
    (let ((done-regexp
           (concat "^\\*\\* \\(" (regexp-opt org-done-keywords) "\\) ")))
      (while (re-search-forward done-regexp nil t)
        (if (>= (time-to-number-of-days
                 (time-subtract (current-time)
                                (apply #'encode-time (org-my-closing-time))))
                org-my-archive-expiry-days)
            (org-archive-subtree))))
    (save-buffer)))

(defalias 'archive-done-tasks 'org-my-archive-done-tasks)