`Date` doesn't look like it gets set anywhere within `spray-client`, and it also doesn't seem to be part of requests coming out of clients using `SprayHttpClient`. It's an optional header for clients, though, so this may not be an issue: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.18

Would not be opposed to allowing `Date` through where appropriate (PUT and POST requests), though the burden would then be on developers to specify an appropriate DateTime.