(require 'time-date) ; part of Gnus

(defun september-citation-line ()
    (insert "On ")
    (insert (int-to-string
	     (-
	      (time-to-days (current-time))
	      (time-to-days (encode-time 0 0 0 01 09 1993)))))
    (insert " September 1993, ")
    (insert (mail-header-from message-reply-headers) 
	    " wrote:\n\n"))

;; make it the default citation line for Gnus' messahe mode
(setq message-citation-line-function 'september-citation-line)