def handle_date_mismatch(self, req):
raise DateMismatchError<EOL>
Handle a request whose date doesn't match the signing key process, by
raising a DateMismatchError.

Overrides the default behaviour of AWS4Auth where the signing key
is automatically regenerated to match the request date

To update the signing key if this is hit, call
StrictAWS4Auth.regenerate_signing_key().