Fix some extra "changed the start time of this event" transactions

Summary: Ref T11326. Since we were missing an `(int)` cast here, the code ended up thinking that changing `12345` to `"12345"` was an edit. It isn't.

Test Plan: Created/edited events, no more extra "changed start time from X to the same X" transaction clutter.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11326

Differential Revision: https://secure.phabricator.com/D16773

src/view/form/control/AphrontFormDateControlValue.php

@@ -182,7 +182,7 @@ final class AphrontFormDateControlValue extends Phobject {
       return null;
     }
 
-    return $datetime->format('U');
+    return (int)$datetime->format('U');
   }
 
   private function getTimeFormat() {