code: private String toLabel(final TimeUnit unit) {
        switch(unit) {
           case DAYS: return "day";
           case HOURS: return "hour";
           case MINUTES: return "minute";
           case SECONDS: return "second";
           case MILLISECONDS: return "ms";
           case MICROSECONDS: return "us";
           case NANOSECONDS: return "ns";
           default: return "";
        }
     }
docs: Converts a time unit to a label.
  @param unit The time unit.
  @return The label.