If you look on the response header of the http://www.yale.edu, there is a field such as
Expires: Sun, 19 Nov 1978 05:00:00 GMT.
curl -I http://www.yale.edu
A couple of years back, this was the main way of specifying when assets expires. Expires is simply a basic date-time stamp. It’s fairly useful for old user agents which still roam unchartered territories. It is however important to note that cache-control headers, max-age and s-maxage still take precedence on most modern systems. It’s however good practice to set matching values here for the sake of compatibility. It’s also important to ensure you format the date properly or it might be considered as expired.
taken from here
After that the response is no longer cached. See here