PHP calculate date difference

date_default_timezone_set("Asia/Calcutta");

function dt_differ($start, $end){
  $start = date("G:i:s:m:d:Y", strtotime($start));
  $date1=explode(":", $start);

  $end  = date("G:i:s:m:d:Y", strtotime($end));
  $date2=explode(":", $end);

  $starttime = mktime(date($date1[0]),date($date1[1]),date($date1[2]),
  date($date1[3]),date($date1[4]),date($date1[5]));
  $endtime   = mktime(date($date2[0]),date($date2[1]),date($date2[2]),
  date($date2[3]),date($date2[4]),date($date2[5]));

  $seconds_dif = $starttime-$endtime;

  return $seconds_dif;
}

PHP