# std::chrono::operator+, std::chrono::operator- (std::chrono::year_month)

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< cpp‎ | chrono‎ | year month

constexpr std::chrono::year_month operator+(const std::chrono::year_month& ym, const std::chrono::years& dy) noexcept; | (1) | (since C++20)
constexpr std::chrono::year_month operator+(const std::chrono::years& dy, const std::chrono::year_month& ym) noexcept; | (2) | (since C++20)
constexpr std::chrono::year_month operator+(const std::chrono::year_month& ym, const std::chrono::months& dm) noexcept; | (3) | (since C++20)
constexpr std::chrono::year_month operator+(const std::chrono::months& dm, const std::chrono::year_month& ym) noexcept; | (4) | (since C++20)
constexpr std::chrono::year_month operator-(const std::chrono::year_month& ym, const std::chrono::years& dy) noexcept; | (5) | (since C++20)
constexpr std::chrono::year_month operator-(const std::chrono::year_month& ym, const std::chrono::months& dm) noexcept; | (6) | (since C++20)
constexpr std::chrono::months operator-(const std::chrono::year_month& ym1, const std::chrono::year_month& ym2) noexcept; | (7) | (since C++20)

1-2) Adds `dy.count()` years to `ym`.
3-4) Adds `dm.count()` months to `ym`.
5) Subtracts `dy.count()` years from `ym`.
6) Subtracts `dm.count()` months from `ym`.
7) Returns the difference in months between the two time points represented by `ym1` and `ym2`.

### Return value

1-2) std::chrono::year_month(ym.year() + dy, ym.month())
3-4) A `year_month` value `z` such that z - ym == dm and z.ok() == true.
5) ym + -dy
5) ym + -dm
7) ym1.year() - ym2.year() + std::chrono::months(int(unsigned(ym1.month())) - int(unsigned(ym2.month())))

### Notes

The result of subtracting two `year_month` values is a duration of type std::chrono::months. This duration unit represents the length of the average Gregorian month (30.436875 days), and the resulting duration bears no relationship to the actual number of days in the time period at issue. For example, the result of 2017y/3 - 2017y/2 is std::chrono::months(1), even though February 2017 only contains 28 days.

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 = ym + decades{1}; // error, ambiguous
    ym = ym + kilomonths{1}; // OK

### Example

| This section is incomplete Reason: no example

### See also

operator+=operator-= | modifies the `year_month` by some number of months or years (public member function)

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