Undefined index: created_at

I'm trying to get the date_diff of queried data using query builder, yet I can't pass the 'create_at' values to count the days difference. Any ideas? Thanks.

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use App\Http\Requests;
use App\Staff;
class StaffController extends Controller
{
    public function index()
    {
        $currentdate = date('Y-m-d  H:i:s');
        $trialusers=Staff::all('id','email','created_at','trial')->where('trial',1);
        $datediff = date_diff($trialusers['created_at'],$currentdate);
        return $datediff;
    }
}

Database

+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| id         | int(11)      | NO   | PRI | NULL    | auto_increment |
| first_name | varchar(255) | NO   |     | NULL    |                |
| last_name  | varchar(255) | NO   |     | NULL    |                |
| email      | varchar(255) | NO   |     | NULL    |                |
| created_at | timestamp    | YES  |     | NULL    |                |
| updated_at | timestamp    | YES  |     | NULL    |                |
| last_login | timestamp    | YES  |     | NULL    |                |
| trial      | tinyint(4)   | NO   |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

via Chebli Mohamed