任意の単位での切り捨て、切り上げ、四捨五入 ref: http://qiita.com/kurogelee/items/69b35e75b10dacf8be8b

round.clj

(defn half [x] (/ x 2))
(defn floor [value unit]
  {:pre [(pos? unit)]}
  (cond
   (Double/isNaN value) value
   (Double/isInfinite value) value
   :else  (let [q (quot value unit)]
            (* unit (if (neg? (rem value unit))
                      (dec q) q)))))

(defn ceil [value unit]
  (- (floor (- value) unit)))

(defn round [value unit]
  (floor (+ value (half unit)) unit))

結果

(floor 3.7 2.5)  ;2.5
(ceil 3.7 2.5)   ;5.0
(round 3.7 2.5)  ;2.5
(round 3.8 2.5)  ;5.0
(floor 12.34 0.1);12.3