functions.clj

(defn delta-periods
  [start end unit span]
  (let [[years units] (map #(delta % start end) [year unit])
        top (per-year unit span)]
    (+ (* top years)
       (int (/ units span)))))

(defn sixteens
  [start end]
  (delta-periods start end day 16))

(defn eights
  [start end]
  (delta-periods start end day 8))

(defn months
  [start end]
  (delta-periods start end month 1))


;; or this:

(defn delta-periods
  [unit span]
  (fn [start end]
    (let [[years units] (map #(delta % start end) [year unit])
          top (per-year unit span)]
      (+ (* top years)
         (int (/ units span))))))

(def sixteens (delta-periods day 16))
(def eights (delta-periods day 8))
(def months (delta-periods month 1))