public final long elapsedTime() {
        if (status == TimeInstrumentStatus.STARTED) {
            return 0;
        } else {
            return stopTime - startTime;
        }
    }
A very basic {@link TimeInstrument} which can be started and stopped
once and only once.

@author Ceki G&uuml;lc&uuml;
slf4j-ext/src/main/java/org/slf4j/profiler/StopWatch.java