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, for caching ('If-Modified-Since').

But what use has it to double the info in a separate Date header?

Answer 1

Per the spec, it is used in age calculations. If you don't know what time the server thinks it is, you won't be able to calculate the "age" of a resource. Here's the relevant text from the spec:

Summary of age calculation algorithm, when a cache receives a response:

age_value
is the value of Age: header received by the cache with this response.

date_value
is the value of the origin server's Date: header

request_time
is the (local) time when the cache made the request that resulted in this cached response

response_time
is the (local) time when the cache received the response

now
is the current (local) time

apparent_age = max(0, response_time - date_value);
corrected_received_age = max(apparent_age, age_value);
response_delay = response_time - request_time;
corrected_initial_age = corrected_received_age + response_delay;
resident_time = now - response_time;
current_age   = corrected_initial_age + resident_time;