explicit constexpr month(unsigned m) noexcept;
constexpr month& month::operator++() noexcept;
constexpr month operator++(int) noexcept;
constexpr month& operator--() noexcept;
constexpr month operator--(int) noexcept;
constexpr month& operator+=(const months& m) noexcept;
constexpr month& operator-=(const months& m) noexcept;
explicit constexpr month::operator unsigned() const noexcept;
constexpr bool month::ok() const noexcept;