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

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;

A reference to this day after modification.

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

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