why does my timer service can't be stopped when revisiting the activity

Question

I'm using a timer service and want to bind/unbind it on a button click in my main activiy. The issue is when I start my timer and exit the activity and come back and attempts to stop it, it doesn’t stop running.

my Timer service code

`public class TimerService extends Service {

   
    private final IBinder binder = new LocalBinder();
    private int second =0, minute=0, hour=0;
    public static String seconds =null, minutes=null, hours=null, finalCounterValue=null;

    public static Timer sessionTimer;

    public class LocalBinder extends Binder {
        TimerService getService() {
            return TimerService.this;
        }
    }

    @Override
    public void onCreate() {
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        return super.onStartCommand(intent, flags, startId);

    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        startTimer();
        return binder;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        sessionTimer.cancel();
        Toast.makeCustomToast(TimerService.this, "Timer is stopped");
    }

    @Override
    public boolean onUnbind(Intent intent) {
        sessionTimer.cancel();
        return super.onUnbind(intent);
    }

    private void startTimer() {
        sessionTimer = new Timer();
        sessionTimer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {

                seconds = String.valueOf(second);
                minutes = String.valueOf(minute);
                hours = String.valueOf(hour);

                if (seconds.length() == 1) {
                    seconds = "0" + seconds;
                }
                if (minutes.length() == 1) {
                    minutes = "0" + minutes;
                }
                if (hours.length() == 1) {
                    hours = "0" + hours;
                }

                second++;

                if (second == 59) {
                    second = 0;
                    minute++;
                }

                if (minute == 59) {
                    minute = 0;
                    second = 0;
                    hour++;
                }
                finalCounterValue = hours + ":" + minutes + ":" + seconds;
                DashboardActivity.txtTime.setText(finalCounterValue);
                Log.d(TAG, "Session timer:"+finalCounterValue);
            }
        }, 0, 1000);
    }

}
`

My main activity this code is in OnCreate() start + stop will call the bind/unbind

`btnStart.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        btnStart = (Button) v;

        if (btnStart.getText().equals("Pause")) {
    
            btnStart.setText("Start");
            btnStart.setBackgroundResource(R.drawable.rounded_red_button);
            stopSessionTimer();
        } else if (btnStart.getText().equals("Start")) {
            startSessionTimer();
            btnStart.setText("Pause");
            btnStart.setBackgroundResource(R.drawable.rounded_pause_button);
        }
    }

});
`

My connection part in Mainactivity

`private final ServiceConnection timerConnection = new ServiceConnection() {
    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {


        timerService = ((TimerService.LocalBinder)service).getService();

       Toast.makeCustomToast(DashboardActivity.this,"Service connected");
    }

    @Override
    public void onServiceDisconnected(ComponentName name) {
        timerService= null;
        Toast.makeCustomToast(DashboardActivity.this,"Service diconnected");
    }
};

void doBindService() {

        if (bindService(new Intent(DashboardActivity.this, TimerService.class),
                timerConnection, Context.BIND_AUTO_CREATE)) {
        bound = true;
    } else {
        Log.e("MY_APP_TAG", "Error: The requested service doesn't " +
                "exist, or this client isn't allowed access to it.");
    }
}

void doUnbindService() {
    if (bound) {
        unbindService(timerConnection);
        bound = false;
    }
}
`

What I want to achieve is I want my main activity to control my timer service (bind/unbind) it on a button click even if I left my activity and came back. Right now it's working fine except if I click on START then leave and come back, I can't stop it.