How can I run stopwatch in background?

Question

I'm working on android app where I need to start/ stop the stopwatch even when app is in background. I'm going to use single button for start/stop and 2 buttons only for starting. I'm calling startStopChronometer() method in onClickListener function of button. Also, chronometer is setting to 0 after we recall the function.

    public void startStopChronometer(){
    if(isStart){
        chronometer.stop();
        isStart = false;
    }else{
        chronometer.setBase(SystemClock.elapsedRealtime());
        chronometer.start();
        isStart = true;
    }
}