Logic checkin method

I built a hotel booking system but i have a problem with the checkin method if I do checkin and checkout, for example:

check in date: 2019-09-17 00:22:00
checkout date: 2019-09-17 12:00:00

the price_total is 0

but if I do checkin and checkout:

checkin date: 2019-09-17 00:22:00
check date: 2019-09-18 12:00:00
the price_total is as I expected

controller :

public function store(Request $request)
{
    $data = new RoomTransaction();
    $ym = Carbon::now()->format('Y/m');
    $row = RoomTransaction::withTrashed()->get()->count() > 0 ? RoomTransaction::withTrashed()->get()->count() + 1 : 1;
    $no_invoice = $ym.'/INV-'.Helper::ref($row);
    $data->invoice_id = $no_invoice;
    $data->guest_id = $request->guest_id;
    $data->adult = $request->adult;
    $data->child = $request->child;
    $data->checkin_date = $request->checkin_date.' '.$request->time_checkin;
    $data->checkout_date = $request->checkout_date.' '.$request->time_checkout;
    $data->room_id = $request->room_id;
    $data->deposite= $request->deposite;
    $data->user_id = Auth::user()->id;
    $data->method = $request->method;
    $data->status = 1;
    $room_id = $data->room_id;
    $room = Room::find($room_id);

    $diff_day = $this->diffday($data->checkin_date,$data->checkout_date);

    $data->price_total = $room->roomtype->price_night * $diff_day;

    if ($data->save()) {

        $room = Room::find($room_id);
        $room->status = 1;
        $room->save();
    }
    return redirect('admin/'.$this->title)->with('success', 'Check-In Success');

}

 private function diffday($checkin,$checkout)
    {
        $checkin = date_create($checkin);
        $checkout = date_create($checkout);
        $interval = date_diff($checkin, $checkout);

       return $interval->format('%a');

    }

I am confused what logic is best for overcoming the problem