template<typename YearType, typename MonthType, typename DayType> 
struct year_month_day_base {
  // types
  typedef YearType  year_type; 
  typedef MonthType month_type;
  typedef DayType   day_type;  

  // construct/copy/destruct
  year_month_day_base(YearType, MonthType, DayType);

  // public member functions

  YearType year;
  MonthType month;
  DayType day;
};