Easy Tutorial step by step with free source code: count days from the moment you open the app until the settled date, in Android Studio version 3.1.3. For this tutorial, I counted the remaining days, hours, minutes and seconds until the Christmas 2018. Create new project with Application name: Days until Christmas; Minimum SDK: API 21Android 5 (Lollipop). Add 2 pictures, one for app background and one for countdown background, in res->drawable folder. In manifest->AndroidManifest.xml write the code:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.app.toyo.daysuntilchristmas">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity"
            android:screenOrientation="landscape">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>
  In app->res->values->strings.xml write the code:
<resources>
    <string name="app_name">Days until Christmas</string>
    <string name="countdown">There are only</string>
    <string name="countdownleft">until Christmas!</string>
</resources>
  In app->res->layout->activity_main.xml write the code:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/background"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/countdown"
        android:textSize="25dp"
        android:textColor="#ffeeee"
        android:textStyle="bold"
        android:layout_above="@+id/relativeLayout"
        android:layout_centerHorizontal="true"
        android:id="@+id/textView1" />

    <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:id="@+id/relativeLayout">

        <LinearLayout
            android:id="@+id/LinearLayout"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:orientation="horizontal">

        <TextView
            android:id="@+id/tvhappyevent"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:paddingLeft="50dp"
            android:gravity="center"
            android:singleLine="true"
            android:text="Merry Christmas 2018!"
            android:textColor="#fff"
            android:textSize="24sp"
            android:textStyle="bold"
            android:visibility="gone" />

        <LinearLayout
            android:id="@+id/LinearLayout10"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/counter"
            android:gravity="center"
            android:orientation="vertical" >

        <TextView
            android:id="@+id/txtTimerDay"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:gravity="center"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:textColor="#4a0000"
            android:textSize="35dp"
            android:textStyle="bold" />

            <TextView
                android:id="@+id/txt_TimerDay"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:gravity="center_horizontal"
                android:text="Days"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:textColor="#fff"
                android:textStyle="bold" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/LinearLayout11"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/counter"
            android:gravity="center"
            android:orientation="vertical" >

        <TextView
            android:id="@+id/txtTimerHour"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:gravity="center"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:textColor="#4a0000"
            android:textSize="35dp"
            android:textStyle="bold" />

            <TextView
                android:id="@+id/txt_TimerHour"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:gravity="center_horizontal"
                android:text="Hours"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:textColor="#fff"
                android:textStyle="bold" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/LinearLayout12"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/counter"
            android:gravity="center"
            android:orientation="vertical" >

        <TextView
            android:id="@+id/txtTimerMinute"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:gravity="center"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:textColor="#4a0000"
            android:textSize="35dp"
            android:textStyle="bold" />

            <TextView
                android:id="@+id/txt_TimerMinute"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:gravity="center_horizontal"
                android:text="Minutes"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:textColor="#fff"
                android:textStyle="bold" />
        </LinearLayout>

        <LinearLayout
            android:id="@+id/LinearLayout13"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:background="@drawable/counter"
            android:gravity="center"
            android:orientation="vertical" >

        <TextView
            android:id="@+id/txtTimerSecond"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:gravity="center"
            android:textAppearance="?android:attr/textAppearanceLarge"
            android:textColor="#4a0000"
            android:textSize="35dp"
            android:textStyle="bold" />

            <TextView
                android:id="@+id/txt_TimerSecond"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:layout_weight="1"
                android:gravity="center_horizontal"
                android:text="Seconds"
                android:textAppearance="?android:attr/textAppearanceSmall"
                android:textColor="#fff"
                android:textStyle="bold" />
        </LinearLayout>
        </LinearLayout>
    </RelativeLayout>

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/countdownleft"
        android:textSize="35dp"
        android:textColor="#ffeeee"
        android:textStyle="bold"
        android:layout_below="@+id/relativeLayout"
        android:layout_centerHorizontal="true"
        android:id="@+id/textView2" />

</RelativeLayout>
  In app->java->MainActivity.java write the code:
package com.app.toyo.daysuntilchristmas;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import java.text.SimpleDateFormat;
import java.util.Date;
import android.os.Handler;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    private TextView txtTimerDay, txtTimerHour, txtTimerMinute, txtTimerSecond;
    private TextView tvEvent;
    private Handler handler;
    private Runnable runnable;

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

        txtTimerDay = (TextView) findViewById(R.id.txtTimerDay);
        txtTimerHour = (TextView) findViewById(R.id.txtTimerHour);
        txtTimerMinute = (TextView) findViewById(R.id.txtTimerMinute);
        txtTimerSecond = (TextView) findViewById(R.id.txtTimerSecond);
        tvEvent = (TextView) findViewById(R.id.tvhappyevent);

        countDownStart();
    }

    public void countDownStart() {
        handler = new Handler();
        runnable = new Runnable() {
            @Override
            public void run() {
                handler.postDelayed(this, 1000);
                try {
                    SimpleDateFormat dateFormat = new SimpleDateFormat(
                            "yyyy-MM-dd");
                    // Please here set your event date//YYYY-MM-DD
                    Date futureDate = dateFormat.parse("2018-12-25");
                    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;
                        txtTimerDay.setText("" + String.format("%02d", days));
                        txtTimerHour.setText("" + String.format("%02d", hours));
                        txtTimerMinute.setText(""
                                + String.format("%02d", minutes));
                        txtTimerSecond.setText(""
                                + String.format("%02d", seconds));

                    } else {
                        tvEvent.setVisibility(View.VISIBLE);
                        tvEvent.setText("The event started!");
                        textViewGone();
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        };
        handler.postDelayed(runnable, 1 * 1000);
}

    public void textViewGone() {
        findViewById(R.id.LinearLayout10).setVisibility(View.GONE);
        findViewById(R.id.LinearLayout11).setVisibility(View.GONE);
        findViewById(R.id.LinearLayout12).setVisibility(View.GONE);
        findViewById(R.id.LinearLayout13).setVisibility(View.GONE);
        findViewById(R.id.textView1).setVisibility(View.GONE);
        findViewById(R.id.textView2).setVisibility(View.GONE);
    }
}
  Run app in mobile device or Emulator (API 21 Android 5) and see how many days are left until Christmas 2018, in format landscape. Follow video tutorial:
Download free Days Until Christmas 2018 application and install it on your mobile device to see how it works: https://android-coffee.com/wp-content/uploads/projecs/DaysUntilChristmas2018.apk
  Download this app for free and install it on your mobile device to see how it works: https://android-coffee.com/wp-content/uploads/projecs/DaysUntilChristmas2018.apk Download project with Source Code from here: https://android-coffee.com/wp-content/uploads/projecs/DaysUntilChristmas2018.7z Please subscribe here to our YouTube channel. Thank you! Check next tutorials to learn all about Android. For further questions leave a message.

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