// MainActivity
protected void onCreate(Bundle savedInstanceState) {
    ...
    startService(new Intent(this, ConnService.class));
    ...
}

// ConnService
public void onCreate() {
    DB = new DBClass();
    DB.connect(this);
    new Thread(new Runnable() {
        @Override
        public void run() {
            while (!finishing) {
                Help.log("Service loop");
                DB.syncWithServer();
                Help.sleep(30 * 1000);
            }
        }
    }).start();
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    return Service.START_STICKY;
}

private void setOnetimeTimer(Context context) {
    AlarmManager am =(AlarmManager)context.getSystemService(Context.ALARM_SERVICE);
    Intent intent = new Intent(context, AlarmReceiver.class);
    PendingIntent pi = PendingIntent.getBroadcast(context, 0, intent, PendingIntent.FLAG_ONE_SHOT);
    am.setExact(AlarmManager.ELAPSED_REALTIME_WAKEUP, System.currentTimeMillis() + (1000 * 30), pi);
}
...
            while (!finishing) {
                Help.log("Service loop");
                DB.syncWithServer();
                setOnetimeTimer(ctx); // <------
            }
...

@Override
public void onTaskRemoved(Intent rootIntent){
    Intent intent = new Intent(getApplicationContext(), this.getClass());
    intent.setPackage(getPackageName());
    PendingIntent pi = PendingIntent.getService(getApplicationContext(), 1, intent, PendingIntent.FLAG_ONE_SHOT);
    AlarmManager am = (AlarmManager) getApplicationContext().getSystemService(Context.ALARM_SERVICE);
    am.set(AlarmManager.ELAPSED_REALTIME,
           SystemClock.elapsedRealtime() + 1000,
           pi);
    super.onTaskRemoved(rootIntent);
}