std::chrono::year_month_day_last::ok
From cppreference.com
                    
                                        
                    < cpp | chrono | year month day last
                    
                                                            
                    | constexpr bool ok() const noexcept; | (since C++20) | |
Checks if *this represents a valid date. Because a year_month_day_last represents the last day of a particular month, it represents a valid date as long as the year and month are valid.
Return value
year().ok() && month().ok()