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Comparing Dates

by Ryan Neufeld

Problem

You need to compare one date to another, or you need to sort a sequence of dates.

Solution

You can compare Java Dates using the compare function.

(defn now [] (java.util.Date.))
(def one-second-ago (now))
(Thread/sleep 1000)

;; Now is greater than (1) one second ago.
(compare (now) one-second-ago)
;; -> 1

;; One second ago is less than (-1) now.
(compare one-second-ago (now))
;; -> -1

;; "Equal" manifests as 0.
(compare one-second-ago one-second-ago)
;; -> 0

Discussion

Why not just compare dates using Clojure’s built-in comparison operators (<=, >, etc.)? The problem with these operators is that they utilize clojure.lang.Numbers and attempt to coerce their arguments to numerical types.

Since regular comparison won’t work, it’s necessary to use the compare function. The compare function takes two arguments and returns a number indicating that the first argument was either less-than (-1), equal-to (0), or greater-than (+1) the second argument.

Clojure’s sort functions use compare under the hood, so no extra work is required to sort a collection of dates.

(def occurrences
  [#inst "2013-04-06T17:40:57.688-00:00"
   #inst "2002-12-25T00:40:57.688-00:00"
   #inst "2025-12-25T11:23:31.123-00:00"])

(sort occurrences)
;; -> (#inst "2002-12-25T00:40:57.688-00:00"
;;     #inst "2013-04-06T17:40:57.688-00:00"
;;     #inst "2025-12-25T11:23:31.123-00:00")

If you’ve been doing more complex work with dates and times and have Joda Time objects in hand, then all of this still applies. If you wish to compare Joda time objects to Java time objects, however, you will have to coerce them to one uniform type using the functions in clj-time.coerce.