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

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

Adds or subtracts d.count() from the weekday value, reducing the result modulo 7 to an integer in the range [0, 6].

1) Performs *this = *this + d;

2) Performs *this = *this - d;

A reference to this weekday after modification.

After a call to one of these functions, ok() is always true if no overflow occurred during the operation.

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