Как исправить ошибку с CountDownTimer в приложении для Android

Я пытаюсь сделать обратный отсчет в днях, часах, минутах, секундах от начальной даты, и по какой-то причине я ошибаюсь в днях, которую не могу найти.

    String givenDateString = "2019-05-15T09:00:00";
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
    try {
        Date mDate = sdf.parse(givenDateString);
         timeInMilliseconds = mDate.getTime();
    } catch (ParseException e) {
        e.printStackTrace();
    }

    new CountDownTimer(timeInMilliseconds, 1000) {
        @Override
        public void onTick(long millisUntilFinished) {
            long day = TimeUnit.MILLISECONDS.toDays(millisUntilFinished);
            millisUntilFinished -= TimeUnit.DAYS.toMillis(day);

            long hour = TimeUnit.MILLISECONDS.toHours(millisUntilFinished);
            millisUntilFinished -= TimeUnit.HOURS.toMillis(hour);

            long minute = TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished);
            millisUntilFinished -= TimeUnit.MINUTES.toMillis(minute);

            long second = TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished);

            prueba.setText("Days: "+day+" Hours: "+hour+" Minutes: "+minute+" Seconds: "+second);
        }

        @Override
        public void onFinish() {
            // What ever you want !
        }

    }.start();

И я получаю этот результат:

Новая ошибка android.view.ViewRootImpl$CalledFromWrongThreadException: только исходный поток, создавший иерархию представлений, может касаться своих представлений.

TextView prueba;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    prueba = findViewById(R.id.prueba);

    Timer timer = new Timer();
    timer.schedule(new TimerTask() {
        @Override
        public void run() {
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
            Date currentDate = null;
            Date destinationDate = null;
            try {
                currentDate = Calendar.getInstance().getTime();
                destinationDate = sdf.parse("2019-05-15T09:00:00");

            } catch (ParseException e) {
                e.printStackTrace();
            }
            calculateDifference(currentDate, destinationDate);
        }
    }, 0, 1000);//Update text every second
}

public void calculateDifference(Date startDate, Date endDate) {
    long different = endDate.getTime() - startDate.getTime();

    long secondsInMilli = 1000;
    long minutesInMilli = secondsInMilli * 60;
    long hoursInMilli = minutesInMilli * 60;
    long daysInMilli = hoursInMilli * 24;

    long days = different / daysInMilli;
    different = different % daysInMilli;

    long hours = different / hoursInMilli;
    different = different % hoursInMilli;

    long minutes = different / minutesInMilli;
    different = different % minutesInMilli;

    long seconds = different / secondsInMilli;

    Log.e("calculation", "Days: " + days + " Hours: " + hours + " Minutes: " + minutes + " Seconds: " + seconds);
    prueba.setText("Days: " + days + " Hours: " + hours + " Minutes: " + minutes + " Seconds: " + seconds);
}