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httpdateheader

What's the rationale behind the HTTP Date header?

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?

Solution

  • 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;