synchronized Duration duration() {
		return isStarted()
			? Duration.ofNanos(_nanos + _nanoClock.getAsLong() - _start)
			: Duration.ofNanos(_nanos);
	}
Return the duration between two consecutive {@link #start()} and
{@link #stop()} calls.

@return the duration between two {@code start} and {@code stop} calls
jenetics/src/main/java/io/jenetics/engine/Timing.java