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

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

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

Notes

See also