无法正确解析Paypal日期

Here are the variables inside my ipn:

$paymentdate = $_POST['payment_date'];
$mdate= date('Y-m-d h:i:s',strtotime($paymentdate));

$paymentdate inserts to table like: Mon May 11 2015 13:01:36 GMT+0800 (Malay Peninsula...

$mdate inserts to table like: 1969-12-31 05:00:00

As you can see $mdate comes out with wrong date and time.

I am trying to get the correct $payment_date to add to the database with the format of Y-m-d h:i:s.

Thanks