constexpr explicit day(unsigned d) noexcept;
constexpr day& operator++() noexcept;
constexpr day operator++(int) noexcept;
constexpr day& operator--() noexcept;
constexpr day operator--(int) noexcept;
constexpr day& operator+=(const days& d) noexcept;
constexpr day& operator-=(const days& d) noexcept;
constexpr explicit operator unsigned() const noexcept;
constexpr bool ok() const noexcept;