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

From cppreference.net
C++
Date and time library
std::chrono::year
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 ; .

Contents

Return value

A reference to this year after modification.

Notes

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

Example

Run this code 
#include <chrono>
#include <iostream>
int main()
{
    using namespace std::literals::chrono_literals;
    std::cout << std::boolalpha;
    std::chrono::year y{2020};
    y += std::chrono::years(12);
    std::cout << (y == 2032y) << ' ';
    y -= std::chrono::years(33);
    std::cout << (y == 1999y) << '\n';
}

Output: 

true true

See also

increments or decrements the year
(public member function)
(C++20)
performs arithmetic on year s
(function)