Some Excel date functions translated to clojure

excel.clj

; Date algorithms from...
; http://rjbaker.org.uk/software/datetime.html

; notleap(y)=[mod(y,4)+2)/3]
(defn NOTLEAP [y]
  (TRUNC (/ (+ (mod y 4) 2) 3)))

; doy(y,m,d) = [275m/9] -       [(m+9)/12] * (1+notleap(y) ) + d -  30
;            = \__p1__/ + (-1 * \___p2___/ * \_____p3____/ ) + d + -30
(defn DAY_OF_YEAR [[y m d]]
  (let [p1 (TRUNC (/ (* 275 m) 9))
        p2 (TRUNC (/ (+ m 9) 12))
        p3 (+ 1 (NOTLEAP y))]
    (+ p1 (* -1 p2 p3) d -30)))

; sch(y,m,d) = trunc(( 1461(y-1900) - 1) / 4) + doy(y,m,d)
;            = trunc(( \____p1____/ - 1) / 4) + \___p2___/
;            = trunc(( \_______p3______/ / 4) + \___p2___/
;            = trunc(( \__________p4_______/) + \___p2___/
(defn DAYS_SINCE_1900 [[y m d]]
  (let [p1 (* 1461 (- y 1900))
        p2 (DAY_OF_YEAR [y m d])
        p3 (- p1 1)
        p4 (/ p3 4)]
    (TRUNC (+ p4 p2))))

; Excel incorrectly treats 1900 as a leap year
(defn EXCEL_DAYS_SINCE_1900 [[y m d]]
  (let [days (DAYS_SINCE_1900 [y m d])]
    (if (>= days 60)
      (+ 1 days)
      days)))

(defn TRUNC [num]
  {:pre [(number? num)]
   :post [()]}
  (if (> num 0) 
    (Math/floor num)
    (Math/ceil num)))

;Eurpoean version of DAYS360 only (Excel quirk free)
;
;360*(y2-y1)+30*(m2-m1)+(d2-d1)
;
(defn DAYS360 
  [[y1 m1 d1] [y2 m2 d2] method]
  {:pre [(= method true)]}
  (let [d1 (min d1 30)
        d2 (min d2 30)]
    (+ (* 360 (- y2 y1))
       (* 30 (- m2 m1))
       (- d2 d1))))