Emacs Org-Mode: Inserts a timestamp into an exported org-file for the time of the export. Original org-file is untouched.

init.el

;; in your init.el

(defun ace/org-export-insert-export-date(&optional backend)
  "Searches for %%DATE_EXPORTED%% in the buffer and replaces all occurrences
   Just put the keyword somewhere in your org buffer and the time stamp of the
   export is inserted in the exported buffer. The original buffer is untouched."
  (goto-char (point-min))
  (while (search-forward "%%DATE_EXPORTED%%" nil t)
    (replace-match (format-time-string "%0e.%0m.%Y %R"))))

;; after org-mode was loaded
(add-hook 'org-export-before-parsing-hook 'ace/org-export-insert-export-date)    

;; rest of your init.el