The term the RFC (RFC 2616, Section 14.18) uses is "MUST" with 3 exceptions (HTTP 100/101 responses, which are message-less; HTTP 500-class errors which are ind

The term the RFC (RFC 2616, Section 14.18) uses is "MUST" with 3 exceptions (HTTP 100/101 responses, which are message-less; HTTP 500-class errors which are indications that the server is malfunctioning and during this malfunction it's inconvenient to generate a date; and finally HTTP servers without clocks), which are all referencing exceptional cases -- in general HTTP/1.1 responses MUST include a Date header from the Origin server, and proxies MUST add the Date header if the Origin server failed to do so (due to 1 or more of the 3 exceptions).