code: public int compareTo(DurationField otherField) {
          long otherMillis = otherField.getUnitMillis();
          long thisMillis = getUnitMillis();
          // cannot do (thisMillis - otherMillis) as can overflow
          if (thisMillis == otherMillis) {
              return 0;
          }
          if (thisMillis < otherMillis) {
              return -1;
          } else {
              return 1;
          }
      }
docs: ------------------------------------------------------------------------