@Override
    public void update() {
        tpf = (getTime() - previousTime) * (1.0f / TIMER_RESOLUTION);
        fps = 1.0f / tpf;
        previousTime = getTime();
    }
Returns the time in seconds. The timer starts
at 0.0 seconds.

@return the current time in seconds
jme3-core/src/main/java/com/jme3/system/NanoTimer.java