Gravity Forms - calculate time between two fields

add_action("gform_pre_submission","pre_submission_handler");

function pre_submission_handler($form) {
  // get input from form
  $date = $_POST["input_1"];
  $start = $_POST["input_11"];
  $end = $_POST["input_5"];

  // convert date and time arrays into datetime formats
  $startdate = date_create_from_format('m/d/Y@h:i a', $date . "@". $start[0].":".$start[1]." ".$start[2]);
  $enddate =   date_create_from_format('m/d/Y@h:i a', $date . "@". $end[0].":".$end[1]." ".$end[2]);

  //convert datetimes into seconds to compare
  $starttime = strtotime($startdate->format('Y-m-d H:i:s'));
  $endtime = strtotime($enddate->format('Y-m-d H:i:s'));

  // check to see if the times span overnight
  if($starttime > $endtime)
    $endtime = strtotime($enddate->format('Y-m-d H:i:s') . " +1 day");

  // perform calculation
  $diff = floor(($endtime - $starttime)/60);

  $_POST["input_50"] = $diff;
}