(use-package :series)

(defun friday13-p (ut)
  (and (= 13 (nth-value 3 (decode-universal-time ut)))
       (= 4 (nth-value 6 (decode-universal-time ut)))))

(let* ((from (get-universal-time))
       (to (encode-universal-time 59 59 23 31 12 2013))
       (uts (choose-if #'friday13-p (scan-range :from from :upto to :by (* 60 60 24)))))
  (iterate ((ut uts)) 
    (format t "~A~%" (metatilities:date-string ut)))
  (collect-length uts))