How can I set comparison datetime in validation laravel?

I use laravel 5.3

My laravel validation like this :

<?php
namespace App\Http\Requests;
use Illuminate\Foundation\Http\FormRequest;
class AddCartRequest extends FormRequest
{
    public function rules()
    {
        return [
            'id'=>'required|numeric',
            ...
            'request_date'=>'required'
        ];
    }
}

If value of request_date is

01-08-2017 13:00:00

I want to add condition like this :

If request date < now + 5 hour, then it will display message : "Request date must be larger 5 hours from now"

How can I do it?