Dave,
I agree that authors would like to expire their documents at some
point in the future, even if those documents are being served
from a cache site. But I don't see how a timestamp is sufficiently
expressive of the reasons an author might have to expire a document.
The reasons might not even be known at the time that the document
is written.
Could VALUE be a boolean expression so that we could do something like:
<META NAME="Expires" VALUE=EXISTSP(<http://original.host/original.file>)>
or:
<META NAME="Expires" VALUE=GT(DATE, "Tue, 04 Jan 1994 14:13:25 GMT")>
There would need to be a way of preventing caches from cacheing the
result of EXISTSP.
Also, clients need to do something reasonable if the boolean
expression cannot be evaluated. Perhaps they could display the page
anyway but with a warning message saying that "the expiry status
of this document is unknown."
John Ellson
AT&T Bell Labs