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

constexpr std::chrono::year& operator+=(const std::chrono::years& y) noexcept;	(1)	(since C++20)
constexpr std::chrono::year& operator-=(const std::chrono::years& y) noexcept;	(2)	(since C++20)

Adds or subtracts y.count() years from the year value.

1) Equivalent to *this = *this + y;

2) Equivalent to *this = *this - y;

A reference to this year after modification.

If the result would be outside the range [-32767, 32767], the actual stored value is unspecified.

operator++operator++(int)operator--operator--(int)	increments or decrements the year (public member function)
operator+operator-	performs arithmetic on `year`s (public member function)