27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.9 Class month_­day [time.cal.md] Overview [time.cal.md.overview]

namespace std::chrono {
  class month_day {
    chrono::month m_;           // exposition only
    chrono::day   d_;           // exposition only

    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 25) and Cpp17LessThanComparable (Table 26) requirements.
month_­day is a trivially copyable and standard-layout class type.