std::chrono::year_month_weekday::ok
From cppreference.com
                    
                                        
                    < cpp | chrono | year month weekday
                    
                                                            
                    | constexpr bool ok() const noexcept; | (since C++20) | |
Checks if this year_month_weekday object represents a valid date.
Return value
true if this year_month_weekday object represents a valid date, that is, year().ok() && month().ok() && weekday_indexed().ok() is true and there are at least index() weekday()s in the specified year and month. Otherwise false.