How to keep the countdown running in background

I'm trying to do a 24-hour countdown but keep running in the background, because I want to send a notification when 5 hours is missing, so far I've been trying to implement a service with a broacast receiver (I do not quite understand that) , however every time the app is closed (deleted from the recent ones) the count stops and restarts when I open the app.

MainActivity:

    package br.com.igoroliv.testecountdown;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "igr";
    private TextView txvcon;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        txvcon = (TextView) findViewById(R.id.txv_tempo);
        startService(new Intent(this, BroadcastService.class));
        Log.i(TAG, "Started service");
    }

    private BroadcastReceiver br = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            updateGUI(intent); // or whatever method used to update your GUI fields
        }
    };

    @Override
    public void onResume() {
        super.onResume();
        registerReceiver(br, new IntentFilter(BroadcastService.COUNTDOWN_BR));
        Log.i(TAG, "Registered broacast receiver");
    }

    @Override
    public void onPause() {
        super.onPause();
        unregisterReceiver(br);
        Log.i(TAG, "Unregistered broacast receiver");
    }

    @Override
    public void onStop() {
        try {
            unregisterReceiver(br);
        } catch (Exception e) {
            // Receiver was probably already stopped in onPause()
        }
        super.onStop();
    }
    @Override
    public void onDestroy() {
        //stopService(new Intent(this, BroadcastService.class));
        //Log.i(TAG, "Stopped service");
        super.onDestroy();
    }

    private void updateGUI(Intent intent) {
        if (intent.getExtras() != null) {
            long millisUntilFinished = intent.getLongExtra("countdown", 0);
            Log.i(TAG, "Countdown seconds remaining: " +  millisUntilFinished / 1000);
            txvcon.setText(""+millisUntilFinished/1000);
        }
    }
}

BroadcastService

package br.com.igoroliv.testecountdown;

import android.app.Service;
import android.content.Intent;
import android.os.CountDownTimer;
import android.os.IBinder;
import android.util.Log;

/**
 * Created by igord on 19/08/2017.
 */

public class BroadcastService  extends Service {

    private final static String TAG = "BroadcastService";

    public static final String COUNTDOWN_BR = "br.com.igoroliv.countdown_br";
    Intent bi = new Intent(COUNTDOWN_BR);

    CountDownTimer cdt = null;

    @Override
    public void onCreate() {
        super.onCreate();

        Log.i(TAG, "Starting timer...");

        cdt = new CountDownTimer(80000, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {

                Log.i(TAG, "Countdown seconds remaining: " + millisUntilFinished / 1000);
                bi.putExtra("countdown", millisUntilFinished);
                sendBroadcast(bi);
            }

            @Override
            public void onFinish() {
                Log.i(TAG, "Timer finished");
            }
        };

        cdt.start();
    }

    @Override
    public void onDestroy() {

        //cdt.cancel();
        //Log.i(TAG, "Timer cancelled");
        super.onDestroy();
    }

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

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }
}