For do my answer and help you, i will completing your code and add a TextView for exemple for put the time on it.

public class MainActivity extends AppCompatActivity {

    // modify here
    private Calendar myTimer = null;
    private TextView myTextView = null;

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

        Button button = (Button) findViewById(R.id.button);
        Button button2 = (Button) findViewById(R.id.button2);

        final TextView lefty = (TextView)findViewById(R.id.left);


        // modify here
        myTextView = (TextView) findViewById(R.id.my_text_view);
        myTimer = Calendar.getInstance();

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Calendar calendar = Calendar.getInstance();
                Long preTime = calendar.getTimeInMillis();
                calendar.add(Calendar.HOUR_OF_DAY, 8);
                Long postTime = calendar.getTimeInMillis();
                Long delay = postTime - preTime;
                Intent intent = new Intent(getApplicationContext(), NotificationReceiver.class);
                PendingIntent pendingIntent = PendingIntent.getBroadcast(getApplicationContext(), 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
                AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
                alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_DAY, pendingIntent);
                CountDownTimer timer = new CountDownTimer(delay, 1) {
                    @Override
                    public void onTick(long millisUntilFinished) {
                        final int seconds = (int) (millisUntilFinished / 1000) % 60;
                        final int minutes = (int) ((millisUntilFinished / (1000 * 60)) % 60);
                        final int hour = (int) ((millisUntilFinished / (1000 * 60 * 60)) % 24);

                        // modify here
                        myTimer.set(Calendar.SECOND, seconds);
                        myTimer.set(Calendar.MINUTE, minutes);
                        myTimer.set(Calendar.HOUR, hour);

                        runOnUiThread(new Runnable() {

                            @Override
                            public void run() {
                                lefty.setText(hour + " : " + minutes + " : " + seconds);
                            }
                        });

                    }

                    @Override
                    public void onFinish() {
                        // TODO Auto-generated method stub

                    }
                };
                timer.start();
            }
        });


        // modify here
        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String myString = new SimpleDateFormat("hh : mm : ss").format(myTimer);
                myTextView.setText(myString);
            }
        });
    }
}