How to validate date using different date format in Laravel using validation rule?

Actually, I want to validate date in laravel validation rules.

'sometimes|date_format:Y-m-d|date_format:Y-m-d H:i:s',

public function rules()
    {
        return [
            'title' => 'required|string|between:3,100',
            'created_at' => 'sometimes|date_format:Y-m-d|date_format:Y-m-d H:i:s',
            'updated_at' => 'required|date_format:Y-m-d',
            'date_of_purchase' => 'required|date_format:Y-m-d',
            'date_of_expiration' => 'required|date_format:Y-m-d'
        ];
    }

Expectations

 'created_at' => 'sometimes|date_format:Y-m-d, Y-M-d H:i:s', 

Is there any way to do this like above mentioned line code. Hope got that.

via Chebli Mohamed