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

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;

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_day. 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 ymd = 2001y/April/10;
ymd += decades{1}; // error, ambiguous
ymd += kilomonths{1}; // OK

See also

adds or subtracts a year_month_day and some number of years or months
(public member function)

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