Simple chronometer in java

Chronometer.java

import java.util.concurrent.TimeUnit;

public class Chronometer implements Comparable<Chronometer> {
	boolean started = false;
	boolean stopped = false;
	long start;
	long end;

	public void start() {
		started = true;
		start = System.nanoTime();
	}

	public void stop() {
		if (!started) {
			throw new IllegalStateException("Can't stop before start");
		}
		stopped = true;
		end = System.nanoTime();
	}

	public long getNanos() {
		if (!started) {
			throw new IllegalStateException("Can't calculate time before being started and stopped");
		}
		if (!stopped) {
			throw new IllegalStateException("Can't calculate time before being stopped");
		}
		return end - start;
	}

	public long getMillis() {
		return TimeUnit.NANOSECONDS.toMillis(getNanos());
	}

	@Override
	public String toString() {
		return getMillis() + " ms";
	}

	public boolean lt(Chronometer o) {
		return compareTo(o) < 0;
	}

	public boolean le(Chronometer o) {
		return compareTo(o) <= 0;
	}

	public boolean gt(Chronometer o) {
		return compareTo(o) > 0;
	}

	public boolean ge(Chronometer o) {
		return compareTo(o) >= 0;
	}

	@Override
	public int compareTo(Chronometer o) {
		return Long.compare(getNanos(), o.getNanos());
	}
}