std::chrono::year_month::operator+=,std::chrono::year_month::operator-= (3) Linux Manual Page

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

Synopsis

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

          performs arithmetic on year_month
operator+ (function)
operator-