Laravel mutate another column to carbon object

User.php

<?php namespace App;

use Illuminate\Auth\Authenticatable;
use Illuminate\Auth\Passwords\CanResetPassword;
use Illuminate\Contracts\Auth\Authenticatable as AuthenticatableContract;
use Illuminate\Contracts\Auth\CanResetPassword as CanResetPasswordContract;
use Illuminate\Database\Eloquent\Model;

/**
 * Class User
 *
 * @package App
 */
class User extends Model implements AuthenticatableContract, CanResetPasswordContract {

    use Authenticatable, CanResetPassword;

    /**
     * The database table used by the model.
     *
     * @var string
     */
    protected $table = 'users';

    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = ['name', 'email', 'password'];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = ['password', 'remember_token'];

    /**
     * The attributes that should be mutated to dates.
     *
     * @var array
     */
    protected $dates = ['last_login'];

}