<?php

namespace App;

use App\Appointment;
use App\Business;
use Carbon\Carbon;

class BookingStrategyDateslot implements BookingStrategyInterface
{
    public function makeReservation(Business $business, $data)
    {
        $data['issuer_id'] = \Auth::user()->id;
        $data['business_id'] = $business->id;
        $data['start_at'] = Carbon::parse($data['_date'] . $business->pref('start_at'), $business->tz)->timezone('UTC')->toDateTimeString();
        $data['duration'] = 0;
        return new Appointment($data);
    }
}

<?php

namespace App;

use App\Appointment;
use App\Business;
use Carbon\Carbon;

class BookingStrategyDateslot implements BookingStrategyInterface
{
    public function makeReservation(Business $business, $data)
    {
        $data['issuer_id'] = \Auth::user()->id;
        $data['business_id'] = $business->id;
        $data['start_at'] = Carbon::parse($data['_date'] . ' 08:00 AM', $business->tz)->timezone('UTC')->toDateTimeString();
        $data['duration'] = 0;
        return new Appointment($data);
    }
}

--- 
+++ 
@@ -12,7 +12,7 @@
     {
         $data['issuer_id'] = \Auth::user()->id;
         $data['business_id'] = $business->id;
-        $data['start_at'] = Carbon::parse($data['_date'] . ' 08:00 AM', $business->tz)->timezone('UTC')->toDateTimeString();
+        $data['start_at'] = Carbon::parse($data['_date'] . $business->pref('start_at'), $business->tz)->timezone('UTC')->toDateTimeString();
         $data['duration'] = 0;
         return new Appointment($data);
     }
Remove time hardcode, get from pref()