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

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< cpp‎ | chrono‎ | month

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

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)operator--operator--(int) | increments or decrements the month (public member function)
operator+operator- (C++20) | performs arithmetic on `month`s (function)

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