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

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< cpp‎ | chrono‎ | year month

constexpr std::chrono::year_month& operator+=(const std::chrono::years& dy) const noexcept; | (1) | (since C++20)
constexpr std::chrono::year_month& operator+=(const std::chrono::months& dm) const noexcept; | (2) | (since C++20)
constexpr std::chrono::year_month& operator-=(const std::chrono::years& dy) const noexcept; | (3) | (since C++20)
constexpr std::chrono::year_month& operator-=(const std::chrono::months& dm) const noexcept; | (4) | (since C++20)

Modifies the time point `*this` represents by the duration `dy` or `dm`.

1) Equivalent to *this = *this + dy;
2) Equivalent to *this = *this + dm;
3) Equivalent to *this = *this - dy;
4) Equivalent to *this = *this - dm;

### Notes

Durations that are convertible to std::chrono::months, but not std::chrono::years, can be directly added to or subtracted from a `year_month`. Durations convertible to std::chrono::years cannot because such durations are also convertible to std::chrono::months, resulting in an ambiguity:

    using namespace std::chrono;

    using decades = duration<int, std::ratio_multiply<std::ratio<10>, years::period>>;
    using kilomonths = duration<int, std::ratio_multiply<std::kilo, months::period>>;

    auto ym = 2001y/April;
    ym += decades{1}; // error, ambiguous
    ym += kilomonths{1}; // OK

### See also

operator+operator- | performs arithmetic on `year_month` (function)

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