std::chrono::day::operator+=, std::chrono::day::operator-= - cppreference.com

From cppreference.com

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 [0255], the actual stored value is unspecified.

Example

#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