Distance beetween hours in laravel 5.1

Ask i have something big problem in my apps, in there i want to search distance hour between my column mulai & durasi the result in the column total, column total is result from distance data column mulai until data column durasi, what idea?

$simpan= array();
    $simpan['nama']=Request::input('nama');
    $simpan['tanggal']=Request::input('tanggal');
    $simpan['deskripsi']=Request::input('deskripsi');
    $simpan['mulai']=Carbon::createFromFormat('H:i', Request::input('mulai'))->toTimeString();
    $simpan['durasi']=Carbon::createFromFormat('H:i', Request::input('durasi'))->toTimeString();
    $simpan['total'] = ???

please help me, thanks.

via Chebli Mohamed