code: int getIndexedField(ReadablePeriod period, int index) {
          int realIndex = iIndices[index];
          return (realIndex == -1 ? 0 : period.getValue(realIndex));
      }
docs: Gets the indexed field part of the period.
  
  @param period  the period to query
  @param index  the index to use
  @return the value of the field, zero if unsupported