我从相对日期和相对日期的最后一天获得特定日期，我如何找到差异？

$result = DB::table('customer')->where('isp',2)->get();
  // start_date is stored date in database
  $date= $result->start_date;
  $dts = Carbon::create($date);

  // in order to get the last day of relative month
  // get the last day of month for instance is equal to 2010-09-30 00:00:00

  // find the difference between in days
}

When I use from diffindays of Laravel the output is 0. Any ideas why?