27 Time library [time]

27.8 The civil calendar [time.cal]

27.8.13 Class year_­month [time.cal.ym] Overview [time.cal.ym.overview]

namespace std::chrono {
  class year_month {
    chrono::year  y_;           // exposition only
    chrono::month m_;           // exposition only

    year_month() = default;
    constexpr year_month(const chrono::year& y, const chrono::month& m) noexcept;

    constexpr chrono::year  year()  const noexcept;
    constexpr chrono::month month() const noexcept;

    constexpr year_month& operator+=(const months& dm) noexcept;
    constexpr year_month& operator-=(const months& dm) noexcept;
    constexpr year_month& operator+=(const years& dy)  noexcept;
    constexpr year_month& operator-=(const years& dy)  noexcept;

    constexpr bool ok() const noexcept;
year_­month represents a specific month of a specific year, but with an unspecified day.
year_­month is a field-based time point with a resolution of months.
year_­month meets the Cpp17EqualityComparable (Table 25) and Cpp17LessThanComparable (Table 26) requirements.
year_­month is a trivially copyable and standard-layout class type.