boost::serialization::save — Function to save nth_day_of_the_week_in_month objects using serialization lib.
// In header: <boost/date_time/gregorian/greg_serialize.hpp> template<typename Archive> ( ar, nkd, );
nth_day_of_the_week_in_month objects are broken down into 3 parts for serialization: the week number, the day of the week, and the month