code: private static DateTime uniqueTimestamp() {
          long currentMillis;
          synchronized (FixedUidGenerator.class) {
              currentMillis = System.currentTimeMillis();
              // guarantee uniqueness by ensuring timestamp is always greater
              // than the previous..
              if (currentMillis < lastMillis) {
                  currentMillis = lastMillis;
              }
              if (currentMillis - lastMillis < Dates.MILLIS_PER_SECOND) {
                  currentMillis += Dates.MILLIS_PER_SECOND;
              }
              lastMillis = currentMillis;
          }
          final DateTime timestamp = new DateTime(currentMillis);
          timestamp.setUtc(true);
          return timestamp;
      }
docs: Generates a timestamp guaranteed to be unique for the current JVM instance.
  @return a {@link DateTime} instance representing a unique timestamp