Below example code will helps you to create Chronometer. For this firstly, In activity_main.xml layout I have added buttons and Chronometer layout. Now See MainActivity, here I have used start(), stop(), SystemClock.elapsedRealtime()), setFormat() methods when you clicks SetFormat button, Time View format will changed dynamically. You can see below example code it clearly describes How to use Chronometer in android

Step(1)activity_main.xml layout-

<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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/btn_start"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:text="Start"
        android:layout_toStartOf="@+id/chronometer" />

    <Button
        android:id="@+id/btn_clear"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:layout_below="@+id/btn_setFormat"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dp"
        android:text="clearFormat" />
    <!-- chronometer with black background and red text color -->
    <Chronometer
        android:id="@+id/chronometer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/btn_clear"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="20dp"
        android:background="#000"
        android:gravity="center"
        android:padding="10dp"
        android:textColor="#f00"
        android:textStyle="bold" />

    <Button
        android:id="@+id/btn_stop"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:text="Stop"
        android:layout_alignBaseline="@+id/btn_start"
        android:layout_alignBottom="@+id/btn_start"
        android:layout_toEndOf="@+id/btn_start"
        android:layout_marginStart="21dp" />

    <Button
        android:id="@+id/btn_restart"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:text="Restart"
        android:layout_below="@+id/btn_start"
        android:layout_alignParentStart="true"
        android:layout_alignEnd="@+id/btn_start" />

    <Button
        android:id="@+id/btn_setFormat"
        android:layout_width="200dp"
        android:layout_height="wrap_content"
        android:text="setFormat"
        android:layout_below="@+id/btn_stop"
        android:layout_toEndOf="@+id/btn_restart"
        android:layout_alignStart="@+id/btn_stop" />

</RelativeLayout>

Step(2)-MainActivity-
 

 protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            // initiate views
            simpleChronometer = (Chronometer) findViewById(R.id.chronometer);
            start = (Button) findViewById(R.id.btn_start);
            stop = (Button) findViewById(R.id.btn_stop);
            restart = (Button) findViewById(R.id.btn_restart);
            setFormat = (Button) findViewById(R.id.btn_setFormat);
            clearFormat = (Button) findViewById(R.id.btn_clear);
            // perform click  event on start button to start a chronometer
            start.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    simpleChronometer.start();
                }
            });

            // Click on stop button to stop the chronometer
            stop.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    simpleChronometer.stop();
                }
            });

            //Click on on restart button to set the base time on chronometer
            restart.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    simpleChronometer.setBase(SystemClock.elapsedRealtime());
                }
            });

            // Click on on set Format button to set the format of chronometer
            setFormat.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    simpleChronometer.setFormat("Time running (%s)");
                }
            });

            //Click on clear format to rest the view format of chronometer
            clearFormat.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    simpleChronometer.setFormat(null);
                }
            });
        }


    }