How to keep a CountDownTimer running even if the app is closed?

Run it in a service as such: use create a broadcast receiver in your activity and have the service send broadcasts.

package com.example.cdt;

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

public class BroadcastService extends Service {

    private final static String TAG = "BroadcastService";

    public static final String COUNTDOWN_BR = "your_package_name.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(30000, 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;
        }
}

From the main activity:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    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);            
    }
}

Note I got this code from How to run CountDownTimer in a Service in Android?, which I modified for my own android countDownTimer.

Download source code from here Android Countdown Timer Run In Background

activity_main.xml

<RelativeLayout android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android">


        <EditText
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/et_hours"
            android:hint="Hours"
            android:inputType="time"
            android:layout_marginRight="5dp"
            />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/btn_timer"
        android:layout_above="@+id/btn_cancel"
        android:text="Start Timer"/>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:id="@+id/btn_cancel"
        android:text="cancel timer"/>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/tv_timer"
        android:layout_centerInParent="true"
        android:textSize="25dp"
        android:textColor="#000000"
        android:text="00:00:00"/>

    </RelativeLayout>

MainActivity.java

package com.countdowntimerservice;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.SharedPreferences;
import android.preference.PreferenceManager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.text.SimpleDateFormat;
import java.util.Calendar;


public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button btn_start, btn_cancel;
    private TextView tv_timer;
    String date_time;
    Calendar calendar;
    SimpleDateFormat simpleDateFormat;
    EditText et_hours;

    SharedPreferences mpref;
    SharedPreferences.Editor mEditor;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        init();
        listener();


    }

    private void init() {
        btn_start = (Button) findViewById(R.id.btn_timer);
        tv_timer = (TextView) findViewById(R.id.tv_timer);
        et_hours = (EditText) findViewById(R.id.et_hours);
        btn_cancel = (Button) findViewById(R.id.btn_cancel);



        mpref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
        mEditor = mpref.edit();

        try {
            String str_value = mpref.getString("data", "");
            if (str_value.matches("")) {
                et_hours.setEnabled(true);
                btn_start.setEnabled(true);
                tv_timer.setText("");

            } else {

                if (mpref.getBoolean("finish", false)) {
                    et_hours.setEnabled(true);
                    btn_start.setEnabled(true);
                    tv_timer.setText("");
                } else {

                    et_hours.setEnabled(false);
                    btn_start.setEnabled(false);
                    tv_timer.setText(str_value);
                }
            }
        } catch (Exception e) {

        }



    }

    private void listener() {
        btn_start.setOnClickListener(this);
        btn_cancel.setOnClickListener(this);

    }

    @Override
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.btn_timer:


                if (et_hours.getText().toString().length() > 0) {

                    int int_hours = Integer.valueOf(et_hours.getText().toString());

                    if (int_hours<=24) {


                        et_hours.setEnabled(false);
                        btn_start.setEnabled(false);


                        calendar = Calendar.getInstance();
                        simpleDateFormat = new SimpleDateFormat("HH:mm:ss");
                        date_time = simpleDateFormat.format(calendar.getTime());

                        mEditor.putString("data", date_time).commit();
                        mEditor.putString("hours", et_hours.getText().toString()).commit();


                        Intent intent_service = new Intent(getApplicationContext(), Timer_Service.class);
                        startService(intent_service);
                    }else {
                        Toast.makeText(getApplicationContext(),"Please select the value below 24 hours",Toast.LENGTH_SHORT).show();
                    }
/*
                    mTimer = new Timer();
                    mTimer.scheduleAtFixedRate(new TimeDisplayTimerTask(), 5, NOTIFY_INTERVAL);*/
                } else {
                    Toast.makeText(getApplicationContext(), "Please select value", Toast.LENGTH_SHORT).show();
                }
                break;


            case R.id.btn_cancel:


             Intent intent = new Intent(getApplicationContext(),Timer_Service.class);
             stopService(intent);

                mEditor.clear().commit();

                et_hours.setEnabled(true);
                btn_start.setEnabled(true);
                tv_timer.setText("");


                break;

        }

    }

    private BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            String str_time = intent.getStringExtra("time");
            tv_timer.setText(str_time);

        }
    };

    @Override
    protected void onResume() {
        super.onResume();
        registerReceiver(broadcastReceiver,new IntentFilter(Timer_Service.str_receiver));

    }

    @Override
    protected void onPause() {
        super.onPause();
        unregisterReceiver(broadcastReceiver);
    }
}

Timer_Service.java

package com.countdowntimerservice;


import android.app.Service;
import android.content.Intent;
import android.content.SharedPreferences;
import android.os.Handler;
import android.os.IBinder;
import android.preference.PreferenceManager;
import android.support.annotation.Nullable;
import android.util.Log;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.TimeUnit;

public class Timer_Service extends Service {

    public static String str_receiver = "com.countdowntimerservice.receiver";

    private Handler mHandler = new Handler();
    Calendar calendar;
    SimpleDateFormat simpleDateFormat;
    String strDate;
    Date date_current, date_diff;
    SharedPreferences mpref;
    SharedPreferences.Editor mEditor;

    private Timer mTimer = null;
    public static final long NOTIFY_INTERVAL = 1000;
    Intent intent;

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

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

        mpref = PreferenceManager.getDefaultSharedPreferences(getApplicationContext());
        mEditor = mpref.edit();
        calendar = Calendar.getInstance();
        simpleDateFormat = new SimpleDateFormat("HH:mm:ss");

        mTimer = new Timer();
        mTimer.scheduleAtFixedRate(new TimeDisplayTimerTask(), 5, NOTIFY_INTERVAL);
        intent = new Intent(str_receiver);
    }


    class TimeDisplayTimerTask extends TimerTask {

        @Override
        public void run() {
            mHandler.post(new Runnable() {

                @Override
                public void run() {

                    calendar = Calendar.getInstance();
                    simpleDateFormat = new SimpleDateFormat("HH:mm:ss");
                    strDate = simpleDateFormat.format(calendar.getTime());
                    Log.e("strDate", strDate);
                    twoDatesBetweenTime();

                }

            });
        }

    }

    public String twoDatesBetweenTime() {


        try {
            date_current = simpleDateFormat.parse(strDate);
        } catch (Exception e) {

        }

        try {
            date_diff = simpleDateFormat.parse(mpref.getString("data", ""));
        } catch (Exception e) {

        }

        try {


            long diff = date_current.getTime() - date_diff.getTime();
            int int_hours = Integer.valueOf(mpref.getString("hours", ""));

            long int_timer = TimeUnit.HOURS.toMillis(int_hours);
            long long_hours = int_timer - diff;
            long diffSeconds2 = long_hours / 1000 % 60;
            long diffMinutes2 = long_hours / (60 * 1000) % 60;
            long diffHours2 = long_hours / (60 * 60 * 1000) % 24;


            if (long_hours > 0) {
                String str_testing = diffHours2 + ":" + diffMinutes2 + ":" + diffSeconds2;

                Log.e("TIME", str_testing);

                fn_update(str_testing);
            } else {
                mEditor.putBoolean("finish", true).commit();
                mTimer.cancel();
            }
        }catch (Exception e){
            mTimer.cancel();
            mTimer.purge();


        }

        return "";

    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.e("Service finish","Finish");
    }

    private void fn_update(String str_time){

        intent.putExtra("time",str_time);
        sendBroadcast(intent);
    }
}

Thanks!