std::chrono::month::operator+=,std::chrono::month::operator-=(3)

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

Synopsis
   constexpr std::chrono::month& operator+=( const (1) (since C++20)
   std::chrono::months& m ) noexcept;
   constexpr std::chrono::month& operator-=( const (2) (since C++20)
   std::chrono::months& m ) noexcept;
   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
// Run this code
 #include <chrono>
 #include <iostream>
 int main()
 {
     std::cout << std::boolalpha;
     std::chrono::month m{6};
     m += std::chrono::months(2);
     std::cout << (m == std::chrono::month(8)) << ' '
               << (m == std::chrono::August) << ' ';
     m -= std::chrono::months(3);
     std::cout << (m == std::chrono::month(5)) << ' '
               << (m == std::chrono::May) << ' ';
     m = std::chrono::October;
     m += std::chrono::months{8}; // ((10 += 8 == 18) % 12) == 6;
     std::cout << (m == std::chrono::June) << ' ';
     m -= std::chrono::months{std::chrono::December - std::chrono::February}; // -= 10
     // (6 -= 10) == -4; -4 % 12 == (12 - 4) == 8
     std::cout << (m == std::chrono::August) << '\n';
 }

Output:
 true true true true true true

See also
   operator++
   operator++(int) increments or decrements the month
   operator-- (public member function)
   operator--(int)
   operator+ performs arithmetic on months
   operator- (function)
   (C++20)