You may want to avoid setting Expires to greater than 1 year, RFC 2616 (HTTP), section 14.21: "To mark a response as 'never expires,' an origin server sends an

You may want to avoid setting Expires to greater than 1 year, RFC 2616 (HTTP), section 14.21: "To mark a response as 'never expires,' an origin server sends an Expires date approximately one year from the time the response is sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one year in the future." If an agent is precisely following the RFC, anything set to more than 1 year in the future is an invalid date and: "HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value '0', as in the past (i.e., 'already expired')."