[Boost-users] plus<boost::gregorian::date> - how to?

I need to be able to either add or substruct boost::gregorian::days from boost::gregorian::date. As far as I understand I have to define plusboost::gregorian::date and miniusboost::gregorian::date and provide function operators for them. My question is: Function operator required in plus and minus template has a form of T operator()(const T& x, const T& y) , but I need to use not only gregorian::date, but gregorian::days as well. Any advise is greatly appreciated.