What's the rationale behind the HTTP Date header?

Question

I have read RFC 2616, but still I wonder, what the Date field is for. There is the Last-Modified field, that actually has a meaning besides just serving metadata, that is, f

Answer 1

The Date is needed only for a better work of Expires header:

Date: Mon, 26 Mar 2012 12:53:02 GMT
Expires: Wed, 25 Apr 2012 12:53:02 GMT

A server or a client may have an incorrect time so client (browser) tries to calculate max age of the resource freshness. That was one of the reasons why the Cache-Control tag was introduced. It uses seconds to expire instead of a fixed time.

I tested Chrome and Firefox and they are fine is response without Date header so it can be safely omitted unless you are still using obsolete Expires header. If the Date is missing it just assumed the same as client's time. It's just insane that in spec the header is mandatory: the date formatting/parsing consumes CPU and network.