Can proxies rewrite Date:?

Henrik Frystyk Nielsen writes:

    John Franks writes:
    
    > What about the date header being part of the data digested for the
    > message-digest.  Is it unsafe to assume that proxies will not mangle
    > the origin-server Date: header?
    
    Caching proxies will do so.

Huh?  Do you mean mean "existing caches do so" or "the HTTP/1.1 spec
allows them to do so?"  The latter is probably false (although not
explicitly so).

Our discussions (in the caching subgroup) have assumed that Date:
is set when the response is generated by the origin server, and
we have used this assumption to define the meaning of Expires:.
If proxies rewrite Date: response headers, we are in some trouble.

-Jeff

P.S.: We're OK if the rewriting results in semantically equivalent
dates (i.e., only a format change), but not if the "mangling" is
more general.