code: private Object readResolve() throws ObjectStreamException {
          // The old index field used to uniquely identify the time unit.
          switch (index) {
          case 6:
              return SECOND;
          case 5:
              return MINUTE;
          case 4:
              return HOUR;
          case 3:
              return DAY;
          case 2:
              return WEEK;
          case 1:
              return MONTH;
          case 0:
              return YEAR;
          default:
              throw new InvalidObjectException("Bad index: " + index);
          }
      }
docs: For backward compatibility only