[time.cal.md.members]

constexpr month_day(const chrono::month& m, const chrono::day& d) noexcept;

Effects: Initializes m_ with m, and d_ with d.

constexpr chrono::month month() const noexcept;

Returns: m_.

constexpr chrono::day day() const noexcept;

Returns: d_.

constexpr bool ok() const noexcept;

Returns: true if m_.ok() is true, 1d <= d_, and d_ is less than or equal to the number of days in month m_; otherwise returns false.

When m_ == February, the number of days is considered to be 29.

30 Time library [time]