Alarmmanager is not working when try get time from sqlLite

I am a newbie in android developer. I tried to make an application that allow user make reminding. The users can add time and their reminding, these data will be saved in sqlLite. And I have some problems when I used Alarmmanage to setup time to display notification for user. This problem is the notification won't display when I closed app or I still in MainActivity. Here is my code:

MainActivity

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(false);


    viewPager = (ViewPager)findViewById(R.id.viewpager);
    setupViewPager(viewPager);
    tabLayout = (TabLayout) findViewById(R.id.tabs);
    tabLayout.setupWithViewPager(viewPager);
    dateFormatter = new SimpleDateFormat("dd-MM-yyyy", Locale.US);
    timeFormatter = new SimpleDateFormat("HH:mm", Locale.US);
    timeHourFormatter = new SimpleDateFormat("HH", Locale.US);
    dbHelper = new DatabaseHelper(MainActivity.this);
    currentDate = Calendar.getInstance();

    aminutes = Integer.parseInt(timeFormatter.format(currentDate.getTime()).substring(3, 5));
    //second = timeFormatter.format(currentDate.getTime()).substring(6, 8);
    //showMessages(timeFormatter.format(currentDate.getTime()) + "second: "+ second);
    if(aminutes > 10){
        timeMinutes = timeHourFormatter.format(currentDate.getTime()) + ":"+aminutes;
    }
    else{
        timeMinutes = timeHourFormatter.format(currentDate.getTime()) + ":0"+aminutes;
    }
    //showMessages(dateFormatter.format(currentDate.getTime())+ " " + timeMinutes);
    arrReminder = dbHelper.getRemindForAlarm(dateFormatter.format(currentDate.getTime()), timeMinutes);
    if(arrReminder.size() >0){
        hours = Integer.parseInt(arrReminder.get(0).get_Time().substring(0, 2));
        minutes = Integer.parseInt(arrReminder.get(0).get_Time().substring(4,5));
        Intent intent = new Intent(MainActivity.this, MyReceiver.class);
        Calendar calendar = Calendar.getInstance();
        calendar.setTimeInMillis(System.currentTimeMillis());

        calendar.set(Calendar.HOUR_OF_DAY, hours);
        calendar.set(Calendar.MINUTE, minutes);
        calendar.set(Calendar.SECOND, 0);
        PendingIntent pendingIntent = PendingIntent.getBroadcast(MainActivity.this, NOW_NOTIFICATION, intent, PendingIntent.FLAG_ONE_SHOT);

        cancelNotification(NOW_NOTIFICATION);
        AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), 2 * 1000, pendingIntent);
    }

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            // start();
            // showMessages(year + "-" + month + "-"+day + " " + hours + ":" + minutes);
            Intent intent = new Intent(MainActivity.this, AddActivity.class);
            startActivity(intent);
           // alarmMamager();
        }
    });

}

public void cancelNotification(int requestCode) {
    try {
        Intent notificationIntent = new Intent(getApplicationContext(), MyReceiver.class);

        pendingIntent = PendingIntent.getBroadcast(getApplicationContext(), requestCode, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
        alarmManager = (AlarmManager) getApplicationContext().getSystemService(Context.ALARM_SERVICE);
        alarmManager.cancel(pendingIntent);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

MyReceiver

 public void onReceive(Context context, Intent intent) {
    long[] v = {500, 1000};
    NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context).setVibrate(v).setSound(RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION)).setSmallIcon(R.drawable.ic_remind).setContentTitle("My notification").setContentText("This is a test message!");
    mBuilder.setAutoCancel(true);
    Intent intent1 = new Intent(context, MainActivity.class);
    intent1.addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP | Intent.FLAG_ACTIVITY_CLEAR_TOP);

    PendingIntent pendingNotificationIntent = PendingIntent.getActivity(context, 0, intent1, PendingIntent.FLAG_UPDATE_CURRENT);
    mBuilder.setContentIntent(pendingNotificationIntent);
    mManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);

    mManager.notify(0, mBuilder.build());
}

When I tried putting default hours and minutes, the problem won't appear. I don'nt know why. Anyone can please help me fix this problem. Thanks you.