std::chrono::month::operator++, std::chrono::month::operator--

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

Adds or subtracts 1 from the month value, reducing the result modulo 12 to an integer in the range [1, 12].

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

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

(none).

1,3) a reference to this month after modification

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

After a call to one of these functions, ok() is always true.

operator+=operator-=	adds or subtracts a number of months (public member function)
operator+operator-	performs arithmetic on `month`s (function)