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

constexpr std::chrono::month& operator+=(const std::chrono::months& m) noexcept;	(1)	(since C++20)
constexpr std::chrono::month& operator+=(const std::chrono::months& m) noexcept;	(2)	(since C++20)

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

1) Performs *this = *this + m;

2) Performs *this = *this - m;

A reference to this month after modification.

After a call to one of these functions, ok() is always true if no overflow occurred during the operation.

operator++operator++(int)operator--operator--(int)	increments or decrements the month (public member function)
operator+operator-	performs arithmetic on `month`s (function)