Allow date classes that extend DateTimeInterface

Author: tplaner

src/Valid.php

@@ -211,7 +211,7 @@ public static function freq($frequency)
      */
     public static function dateTimeObject($dateTime)
     {
-        return (is_object($dateTime) && $dateTime instanceof \DateTime);
+        return (is_object($dateTime) && ($dateTime instanceof \DateTimeInterface));
     }
 
     /**

src/When.php

@@ -305,7 +305,7 @@ public function rrule($rrule)
         return $this;
     }
 
-    public function occursOn(DateTime $date)
+    public function occursOn(\DateTimeInterface $date)
     {
         if (!Valid::dateTimeObject($date))
         {