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

constexpr std::chrono::day& operator++() noexcept;	(1)	(since C++20)
constexpr std::chrono::day operator++(int) noexcept;	(2)	(since C++20)
constexpr std::chrono::day& operator--() noexcept;	(3)	(since C++20)
constexpr std::chrono::day operator--(int) noexcept;	(4)	(since C++20)

Adds or subtracts 1 from the day value.

1-2) Performs *this += std::chrono::days{1};

3-4) Performs *this -= std::chrono::days{1};

(none).

1,3) a reference to this day after modification

2,4) a copy of the day made before modification

If the result would be outside the range [0, 255], the actual stored value is unspecified.

operator+=operator-=	adds or subtracts a number of days (public member function)
operator+operator-	adds or subtracts a number of days and a `day`, or find the difference between two `day`s (function)