Update a field (#+LAST_MODIFIED: ) at save

How To
19

I’m trying @alexv suggestion which works very well with default time-stamp-format:

  (add-hook 'org-mode-hook
    (lambda () (add-hook 'before-save-hook 'time-stamp nil 'local)))

  (add-hook 'org-mode-hook
    (lambda ()
      (set (make-local-variable 'time-stamp-pattern)
     	   "7/^#\\+EDITED: %%$"
	   )))

but does not work if changing time-stamp-format in last but one line:

   	        "7/^#\\+EDITED: [%04y-%02m-%02d %3a %02H:%02M]"

Any idea how to make it work?