code: public String format(Date ref, Date then, long precision)
      {
          List<Duration> durations = DurationHelper.calculatePreciseDuration(ref, then, prettyTime.getUnits());
          List<Duration> retained = retainPrecision(durations, precision);
          return retained.isEmpty() ? "" : prettyTime.format(retained);
      }
docs: Convenience format method for precise durations.
  
  @param ref
  The date of reference.
  @param then
  The future date.
  @param precision
  The precision to retain in milliseconds.
  @return a relative format date as text representation or an empty string
  if no durations are retained