Gravity Forms Calculate Time PHP 5.2

add_action("gform_pre_submission_2","pre_submission_handler");
function pre_submission_handler($form) {
    $format = '%r';
    $start = $_POST["input_1"];
    $end = $_POST["input_2"];

    $startA = strtoupper($start[2]);
    $endP = strtoupper($end[2]);

    $startdate = $start[0].':'.$start[1].' '.$startA;
    $enddate = $end[0].':'.$end[1].' '.$endP;

    $starttime = strptime($startdate, $format);
    $endtime = strptime($enddate, $format);

    $hours = (($endtime['tm_min'] + ($endtime['tm_hour'] * 60)) / 60) - (($starttime['tm_min'] + ($starttime['tm_hour'] * 60)) / 60);
    $_POST["input_3"] = $hours;
}