valid-date.clj

(defn divisible-by? [x y]
  (zero? (rem y x)))

(defn leap? [year]
  (condp divisible-by? year
    400 true
    100 false
    4 true
    false))

(def days [31 28 31 30 31 30 31 31 30 31 30 31])
(def max-days {false days true (assoc-in days [1] 29)})

(defn max-days-for [year month]
  (get-in max-days [(leap? year) (dec month)] -1))

(defn valid-date? [year month day]
  (when-let [max-days-for-month (max-days-for year month)]
    (<= 1 day max-days-for-month)))