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std::chrono::operator+, std::chrono::operator- (std::chrono::day)

constexpr std::chrono::day operator+(const std::chrono::day& d, const std::chrono::days& ds) noexcept;	(1)	(since C++20)
constexpr std::chrono::day operator+(const std::chrono::days& ds, const std::chrono::day& d) noexcept;	(2)	(since C++20)
constexpr std::chrono::day operator-(const std::chrono::day& d, const std::chrono::days& ds) noexcept;	(3)	(since C++20)
constexpr std::chrono::days operator-(const std::chrono::day& x, const std::chrono::day& y) noexcept;	(4)	(since C++20)

1-2) Adds ds.count() days to d.

3) Subtracts ds.count() days from d.

4) Calculate the difference, in days, between two day x and y.

1-2) std::chrono::day(unsigned(d) + ds.count())

3) std::chrono::day(unsigned(d) - ds.count())

4) std::chrono::days(int(unsigned(x)) - int(unsigned(y)))

For (1-3), if the result would be outside the range [0, 255], the actual stored value is unspecified.

operator++operator++(int)operator--operator--(int)	increments or decrements the day (public member function)
operator+=operator-=	adds or subtracts a number of days (public member function)