29 Time library [time]

29.8 The civil calendar [time.cal]

29.8.9 Class month_day [time.cal.md] Member functions [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.