std::chrono::month_weekday::ok
From cppreference.com
                    
                                        
                    < cpp | chrono | month weekday
                    
                                                            
                    | constexpr bool ok() const noexcept; | (since C++20) | |
Checks if the contained month and weekday_indexed objects are valid.
Return value
month().ok() && weekday_indexed().ok()