(princ (loop for y from 2008 to 2013 sum
         (loop for m from 1 to 12
           as x = (encode-universal-time 0 0 0 13 m y)
           as a = (and (< (get-universal-time) x)
                       (= (elt (multiple-value-list (decode-universal-time x)) 6) 4))
           count a if a do (format t "~D-~D-13~%" y m))))