std::chrono::day::operator+=, std::chrono::day::operator-=
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | 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 twodays(function) |