std::chrono::month::operator+=, std::chrono::month::operator-=
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                    |   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;
Return value
A reference to this month after modification.
Notes
After a call to one of these functions, ok() is always true if no overflow occurred during the operation.
Example
| This section is incomplete Reason: no example  | 
See also
|    increments or decrements the month   (public member function)  | |
   performs arithmetic on months (function)  |