A computer method and related apparatus for delivering content files to a client computer are disclosed wherein a request for a content file on a content server is received from a client; a future time period during which the content file will be available on a cache server is determined; and a rewritten uniform resource locator comprising time period availability information, or a link to a file comprising the time period availability information, is returned from the server to the client. The client can use the information to obtain the content file in the determined future time period.