namespace std::chrono {
  class month_day {
    chrono::month m_;           
    chrono::day   d_;           
  public:
    month_day() = default;
    constexpr month_day(const chrono::month& m, const chrono::day& d) noexcept;
    constexpr chrono::month month() const noexcept;
    constexpr chrono::day   day()   const noexcept;
    constexpr bool ok() const noexcept;
  };
}
month_day represents a specific day of a specific month,
but with an unspecified year
.  month_day meets the 
Cpp17EqualityComparable (Table 
28)
and 
Cpp17LessThanComparable (Table 
29) requirements
. month_day is a trivially copyable and standard-layout class type
.