private String getDuration() {

		if (this.start == 0)
			return "not started";
		if (this.stop == 0)
			return "not finished";

		// calculate duration and matching unit
		double duration = this.stop - this.start;
		String unit = "ms";
		if (duration > 1000) {
			duration /= 1000;
			unit = "sec";

			if (duration > 60) {
				duration /= 60;
				unit = "min";

				if (duration > 60) {
					duration /= 60;
					unit = "h";
				}
			}
		}

		return String.format("%.2f %s", duration, unit);
	}