code: public String format(final Duration duration)
     {
        if (duration == null)
           return format(now());
  
        TimeFormat format = getFormat(duration.getUnit());
        String time = format.format(duration);
        return format.decorate(duration, time);
     }
docs: Format the given {@link Duration} object, using the {@link TimeFormat} specified by the {@link TimeUnit} contained
  within. If the given {@link Duration} is <code>null</code>, the current value of
  {@link System#currentTimeMillis()} will be used instead.
  
  @param duration the {@link Duration} to be formatted
  @return A formatted string representing {@code duration}