std::chrono::day::operator+=, std::chrono::day::operator-=
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                    |   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 [0, 255], the actual stored value is unspecified.
Example
| This section is incomplete Reason: no example  | 
See also
|    increments or decrements the day   (public member function)  | |
   adds or subtracts a number of days and a day, or find the difference between two days  (function)  |