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].
*this = *this + d;
*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.
increments or decrements the weekday (public member function) 

performs arithmetic on weekday s (function) 
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