That's what I thought at first. It turns out that the "Date:" header
is very weakly defined in the RFCs and is likely to contain just about
anything from the date the document was created to the date it is sent
from the server. In other words, if "Date:" were used consistently, I
would agree that what I have proposed is redundant.
I'm not sure that it would be safe to build on "Date:". That's why I am
proposing a new header line with a very specific definition.