std::chrono::weekday::operator+=, std::chrono::weekday::operator-=
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                    | 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;
Return value
A reference to this weekday after modification.
Notes
After a call to one of these functions, ok() is always true if no overflow occurred during the operation.
Example
| This section is incomplete Reason: no example | 
See also
| increments or decrements the weekday (public member function) | |
| performs arithmetic on weekdays(function) |