how to get date difference

In laravel i am using this :

@php
 $user_id = Session::get('agentid');
 $res = DB::table('tbl_agent_details as tad')
       ->select('tad.licence_exp_date as date')
       ->where('tad.agent_id','=',$user_id)
       ->get();
$date_time=$res[0]->date;
$date = new DateTime($date_time);
$now = date('Y-m-d');
$dayss_left=$date->diff($now)->format("%d");
@endphp

//In $date_time i am getting :"2018-06-28"
//And In $now getting:        "2018-02-25"  

Its returning me erroe like:

DateTime::diff() expects parameter 1 to be DateTimeInterface, string given

What i am doing wrong here can anyone please tell i want days only

via Chebli Mohamed