XML : Usort Date Xml PHP

I'm trying to order my XML by date but code doesn't work well. Here is my code

  $xml = simplexml_load_file ('http://xml.dsautoestoque.com/?l=09086369000133');              $arr=array();              foreach($xml->veiculo as $aTask)              {                  $arr[]=$aTask;                }                usort($arr, function($a, $b) {                $ad = new DateTime($a['cadastro']);                $bd = new DateTime($b['cadastro']);                  if ($ad == $bd) {                  return 0;                }                  return $ad > $bd ? 1 : -1;              });    

I did like this

      foreach($arr as $aTask)    

{

  $data1 = $aTask->cadastro;    

}

But is not returning by date, it is returning this 03/08/2015 18:22 24/11/2015 11:16 22/10/2015 17:36