Subject: A question about RFC7232#2.2.2

From: Michael Lee <[email-masked]>
Date: Sat, 3 Dec 2016 03:11:38 +0800

I am currently working on a tutorial of HTTP/1.1 caching, but I got 
stuck with the following statement in RFC7232 section 2.2.2:

" This method relies on the fact that if two different responses were 
sent by the origin server during the same second, but both had the same 
Last-Modified time, then at least one of those responses would have a 
Date <http://httpwg.org/specs/rfc7231.html#header.date> value equal to 
its Last-Modified time. The arbitrary 60-second limit guards against the 
possibility that the Date and Last-Modified values are generated from 
different clocks or at somewhat different times during the preparation 
of the response. An implementation /MAY/ use a value larger than 60 
seconds, if it is believed that 60 seconds is too short."

I don't understand why under the circumstance above, at least one of 
those responses would have a Date value equal to its Last-Modified time.

And what's the point of ensuring a 60 seconds gap between the 
Last-Modified time and Date?

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