Just as curiosity, I was looking the [`DateTime` documentation](http://us2.php.net/manual/en/class.datetime.php) today and they have the constant for the RFC formats.

Form the [cookie RFC](http://tools.ietf.org/html/rfc2616#section-3.3.1), it could be ISO 1123 (most used and the same you used), RFC 1036 or asctime().