code: private int monthLength(int month) {
          int year = internalGet(YEAR);
          if (internalGetEra() == BCE) {
              year = 1 - year;
          }
          return monthLength(month, year);
      }
docs: Returns the length of the specified month in the year provided
  by internalGet(YEAR).
  
  @see #isLeapYear(int)