diff --git a/Timer/Timer.php b/Timer/Timer.php
index 37ba982..ca90306 100644
--- a/Timer/Timer.php
+++ b/Timer/Timer.php
@@ -293,7 +293,7 @@ private function checkTime($intervals = array())
     /**
      * Check date string or object and convert if necessary.
      *
-     * @param mixed $date
+     * @param string|null $date
      * @return \DateTime
      */
     private function convertDate($date)