code: 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;
      }
docs: A simple check to see if the given month is part of the extended months.
  
  @param month the month to check
  @return true if the given month is part of the extended months
  @see #setExtendedViewUnit(ViewUnit)
  @see #setExtendedUnitsBackward(int)
  @see #setExtendedUnitsForward(int)