https://www.hackerrank.com/challenges/time-conversion/submissions/code/15384652

timeconv.clj

(require '[clojure.string :as string])

(defn- str->int [s]
  (Integer/parseInt s))

(defn- parse-time [timestr]
  (let [[hours mins secs-and-ampm] (string/split timestr #":" 3)
        secs (subs secs-and-ampm 0 2)
        ampm (subs secs-and-ampm 2)]
    {:hours (str->int hours)
     :minutes (str->int mins)
     :seconds (str->int secs)
     :ampm (string/lower-case ampm)}))

(defn- am? [tm]
  (= "am" (:ampm tm)))

(defn- pm? [tm]
  (= "pm" (:ampm tm)))

(defn- format-time [tm]
  (format "%02d:%02d:%02d" (:hours tm) (:minutes tm) (:seconds tm)))

(defn print-time [tm]
  (println (format-time tm)))

(defn -main [& args]
  (let [tm (parse-time (read-line))
        hours (:hours tm)]
    (cond
      (and (am? tm) (= 12 hours)) (print-time (assoc tm :hours 0))
      (and (pm? tm) (< hours 12)) (print-time (assoc tm :hours (+ hours 12)))
      :else (print-time tm))))