code: public List<TimeUnit> getUnits()
     {
        if (cachedUnits == null) {
           List<TimeUnit> result = new ArrayList<TimeUnit>(units.keySet());
           Collections.sort(result, new TimeUnitComparator());
           cachedUnits = Collections.unmodifiableList(result);
        }
  
        return cachedUnits;
     }
docs: Get the unmodifiable {@link List} of the current configured {@link TimeUnit} instances in calculations.