The difference? Assuming that you have two date time strings(or objects, but objects cannot come directly from post vars so I assume its formatted string like xx:yy am|pm), you can do the following:
$mytimein = new DateTime($_POST['tmestart']);
$mytimeout = new DateTime($_POST['tmeend']; );
$mytimediff = $mytimeout->diff($mytimein);
echo $mytimediff->format("h:i");