29 Time library [time]

29.8 The civil calendar [time.cal]

29.8.13 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 public: 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 28) and Cpp17LessThanComparable (Table 29) requirements.
year_month is a trivially copyable and standard-layout class type.