pre_submission_handler

<?php

add_action("gform_pre_submission","pre_submission_handler");

function pre_submission_handler($form) {
  // get input from form
  $date     =   $_POST["input_1"]; //GF 'date picker' using mm/dd/yyyy
  $start    =   $_POST["input_2"]; //GF 'timefield' using military time
  $end      =   $_POST["input_3"]; //GF 'timefield' using military time

  // convert date and time arrays into datetime formats
  $startdate = new DateTime( $date . ' ' . $start[0] . ':' . $start[1] ) ;

  $enddate = new DateTime( $date . ' ' . $end[0] . ':' . $end[1] ) ;

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

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

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

  $_POST["input_7"] = $diff;
}