A question about RFC7232#2.2.2


#1

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?