Ryan Singel of Wired News wonders if RSS readers will clog Web servers.
The solution is HTTP status code 304 Not Modified. See section 10.3.5 of the HTTP 1.1 specification for more details.
“If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.” Date is a new field value. If no date found (clockless origin server) you use client date.