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

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< cpp‎ | chrono‎ | year month day

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

For durations that are convertible to both std::chrono::years and std::chrono::months, the `years` overloads (1,3) are preferred if the call would otherwise be ambiguous.

### Example

Run this code

    #include <iostream>
    #include <cassert>
    #include <chrono>

    int main()
    {
        std::cout << std::boolalpha;

        constexpr auto monthsInYear {12};
        auto ymd {std::chrono::day(1)/std::chrono::July/2020};

        ymd -= std::chrono::years(10);
        std::cout << (ymd.month() == std::chrono::July) << ' '  // true
                  << (ymd.year() == std::chrono::year(2010)) << ' ';  // true

        ymd += std::chrono::months(10 * monthsInYear + 11);
        std::cout << (ymd.month() == std::chrono::month(6)) << ' '  // true
                  << (ymd.year() == std::chrono::year(2021)) << ' ';  // true

        // Handling the ymd += months "overflow" case.

        ymd = std::chrono::May/31/2021;  // ok
        std::cout << ymd.ok() << ' ';  // true
        ymd += std::chrono::months{1};  // bad date: June has only 30 days
        std::cout << ymd.ok() << ' ';  // false
        assert(ymd == std::chrono::June/31/2021);

        // Snap to the last day of the month, June 30:
        const auto ymd1 = ymd.year()/ymd.month()/std::chrono::last;
        std::cout << ymd1.ok() << ' '; // true
        assert(ymd1 == std::chrono::June/30/2021);

        // Overflow into the next month, July 1 (via converting to/from sys_days):
        const std::chrono::year_month_day ymd2 = std::chrono::sys_days{ymd};
        std::cout << ymd2.ok() << ' '; // true
        assert(ymd2 == std::chrono::July/1/2021);
    }

Output:

    true true true true true false true true

### See also

operator+operator- (C++20) | adds or subtracts a `year_month_day` and some number of years or months (function)

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