# std::chrono::day::operator+=, std::chrono::day::operator-=

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< cpp‎ | chrono‎ | day

constexpr std::chrono::day& operator+=( const std::chrono::days& d ) noexcept; | (1) | (since C++20)
constexpr std::chrono::day& operator-=( const std::chrono::days& d ) noexcept; | (2) | (since C++20)

Adds or subtracts d.count() days from the day value.

1) Equivalent to *this = *this + d;
2) Equivalent to *this = *this - d;

### Return value

A reference to this `day` after modification

### Notes

If the result would be outside the range [0, 255], the actual stored value is unspecified.

### Example

Run this code

    #include <cassert>
    #include <chrono>

    int main()
    {
        std::chrono::day d {15};

        d += std::chrono::days(2);
        assert(d == std::chrono::day(17));

        d -= std::chrono::days{3};
        assert(d == std::chrono::day(14));
    }

### See also

operator++operator++(int)operator--operator--(int) | increments or decrements the day (public member function)
operator+operator- (C++20) | adds or subtracts a number of days and a `day`, or find the difference between two `day`s (function)

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