docs: public final boolean isExtendedMonth(YearMonth month) {
          if (month != null) {
              YearMonth extendedStart = getExtendedStartMonth();
              if ((month.equals(extendedStart) || month.isAfter(extendedStart)) && month.isBefore(getStartMonth())) {
                  return true;
              }
  
              YearMonth extendedEnd = getExtendedEndMonth();
              if ((month.equals(extendedEnd) || month.isBefore(extendedEnd)) && month.isAfter(getEndMonth())) {
                  return true;
              }
          }
          return false;
      }