This will give you the same result:
//MySQL datetime format (same as NOW())
$date = date("Y-m-d H:i:s");
$query = $db->prepare('INSERT INTO pro(reference, date_add) VALUES ( :reference, :date ) ');
$query->execute(array(
':reference' => $_POST['reference'],
':date' => $date
));
$_SESSION['reference'] = $_POST['reference'];
$_SESSION['date_add'] = $date