Laravel: how to set date format on model attribute casting?

After laravel 5.6,

You can individually customize the format of Eloquent date and datetime casting:

protected $casts = [
    'birthday'  => 'date:Y-m-d',
    'joined_at' => 'datetime:Y-m-d H:00',
];

This format is used when the model is serialized to an array or JSON data