About HTTP 1.1 Corrected_initial_age

First, we cite the section "13.2.3 Age Calculations" in Hypertext Transfer Protocol -- HTTP/1.1 [Request for Comments: 2068].

Summary of age calculation algorithm, when a cache receipts 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 resolved in this cached response
* response_time
* is the (local) time when the cache received the
* response
* now
* is the current(local) time
*/
maintain_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;

Data related to age include the transmission time (date_value) and age value (age_value) recorded on the server clock.
If the client's clock and the server's clock do not deviate too much, the reception time (response_time) - the transmission time (date_value) will determine the elapsed time, but if the clock is off (the server's clock is running), the same discrepancy may occur.The first formula (below) is used to avoid such inconsistencies (to be 0 instead of negative time).

apparent_age=max(0,response_time-date_value);

The age value (age_value) is an HTTP/1.1 feature, so if there is an HTTP/1.0 machine in the middle, it will be inaccurate (there is no age_value item in HTTP/1.0, so it will probably be 0), so the following formula appears:

corrected_received_age=max(apparent_age,age_value);

In a multi-machine networking system, you can't expect a clean environment where all machines meet specifications and all machines' clocks are synchronized, so you may find yourself struggling to get even a little "better" elapsed time.