26 Time library [time]

26.8 The civil calendar [time.cal]

26.8.3 Class day [time.cal.day]

26.8.3.1 Overview [time.cal.day.overview]

namespace std::chrono {
  class day {
    unsigned char d_;           // exposition only
  public:
    day() = default;
    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;
  };
}
day represents a day of a month.
It normally holds values in the range 1 to 31, but may hold non-negative values outside this range.
It can be constructed with any unsigned value, which will be subsequently truncated to fit into day's unspecified internal storage.
day is Cpp17EqualityComparable (Table 23) and Cpp17LessThanComparable (Table 24), and participates in basic arithmetic with days objects, which represent a difference between two day objects.
day is a trivially copyable and standard-layout class type.