diff --git a/lib/crontab/date_checker.ex b/lib/crontab/date_checker.ex
index ef0a4c2..8e85819 100644
--- a/lib/crontab/date_checker.ex
+++ b/lib/crontab/date_checker.ex
@@ -7,6 +7,8 @@ defmodule Crontab.DateChecker do
 
   alias Crontab.DateHelper
 
+  @type date :: NaiveDateTime.t() | DateTime.t()
+
   @doc """
   Check a condition list against a given date.