我正在尝试在我的应用程序中创建一个带有特定日期的倒计时计时器

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout

    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.project.MainActivity"
    android:id="@+id/drawer_layout">

    <LinearLayout

        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:orientation="vertical"
        android:weightSum="1">

        <include
            layout="@layout/toolbar_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content" />

        <TextView
            android:id="@+id/countdown"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Next Event:"
            android:textStyle="bold"
            android:textSize="25dp" />
    </LinearLayout>

    <android.support.design.widget.NavigationView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/navigation_view"
        android:layout_gravity="start"
        app:menu="@menu/drawer_menu"
        app:headerLayout="@layout/navigation_drawer_header"
        ></android.support.design.widget.NavigationView>

</android.support.v4.widget.DrawerLayout>

    private TextView txtCountdownEvent;
    private Handler handler;
    private Runnable runnable;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
        txtCountdownEvent = (TextView) findViewById(R.id.countdown);

        actionBarDrawerToggle = new

                ActionBarDrawerToggle(this, drawerLayout, toolbar, R.string.drawer_open, R.string.draw_close);

        drawerLayout.addDrawerListener(actionBarDrawerToggle);

        countDownStart();
    }



    protected void onPostCreate(Bundle savedInstanceState) {
        super.onPostCreate(savedInstanceState);
        actionBarDrawerToggle.syncState();

    }
    public void countDownStart() {
        handler = new Handler();
        runnable = new Runnable() {
            @RequiresApi(api = Build.VERSION_CODES.N)
            @Override
            public void run() {
                handler.postDelayed(this, 1000);
                try {
                    SimpleDateFormat dateFormat = null;
                    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.N) {
                        dateFormat = new SimpleDateFormat(
                                "yyyy-MM-dd");
                    }
// Please here set your event date//YYYY-MM-DD
                    Date futureDate = dateFormat.parse("2017-09-23");
                    Date currentDate = new Date();
                    if (!currentDate.after(futureDate)) {
                        long diff = futureDate.getTime()
                                - currentDate.getTime();
                        long days = diff / (24 * 60 * 60 * 1000);
                        diff -= days * (24 * 60 * 60 * 1000);
                        long hours = diff / (60 * 60 * 1000);
                        diff -= hours * (60 * 60 * 1000);
                        long minutes = diff / (60 * 1000);
                        diff -= minutes * (60 * 1000);
                        long seconds = diff / 1000;
                        txtCountdownEvent.setText("" + String.format("%02d", days, hours, minutes, seconds));

                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        };
        handler.postDelayed(runnable, 1 * 1000);

是因为我无法在一个文本视图中显示天、小时、分钟和秒吗。我遇到的问题是,当我运行应用程序时,计时器不会显示在应用程序上。非常感谢您的帮助。