gistfile1.clj

(defmulti group-data :group-by)
(clojure.tools.macro/macrolet [(impl [key expr]
                                     `(defmethod group-data ~key [kvs#]
                                        (->> (:data kvs#)
                                             (group-by (fn [kv#]
                                                         (let [~'date (:time kv#)]
                                                           ~expr))))))]
  (impl :day (str (month date) "-" (day date) "-" (year date)))
  (impl :month (str (month date) "-" (year date)))
  (impl :year (year date)))