activity_main.xmlMainActivity.java
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:gravity="center"
    android:orientation="vertical"
    tools:context="com.codinginflow.chronometerexample.MainActivity">

    <Chronometer
        android:id="@+id/chronometer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="30sp" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="startChronometer"
        android:text="Start" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="pauseChronometer"
        android:text="Pause" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="resetChronometer"
        android:text="Reset" />

</LinearLayout>

package com.codinginflow.chronometerexample;

import android.os.SystemClock;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Chronometer;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {
    private Chronometer chronometer;
    private long pauseOffset;
    private boolean running;

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

        chronometer = findViewById(R.id.chronometer);
        chronometer.setFormat("Time: %s");
        chronometer.setBase(SystemClock.elapsedRealtime());

        chronometer.setOnChronometerTickListener(new Chronometer.OnChronometerTickListener() {
            @Override
            public void onChronometerTick(Chronometer chronometer) {
                if ((SystemClock.elapsedRealtime() - chronometer.getBase()) >= 10000) {
                    chronometer.setBase(SystemClock.elapsedRealtime());
                    Toast.makeText(MainActivity.this, "Bing!", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

    public void startChronometer(View v) {
        if (!running) {
            chronometer.setBase(SystemClock.elapsedRealtime() - pauseOffset);
            chronometer.start();
            running = true;
        }
    }

    public void pauseChronometer(View v) {
        if (running) {
            chronometer.stop();
            pauseOffset = SystemClock.elapsedRealtime() - chronometer.getBase();
            running = false;
        }
    }

    public void resetChronometer(View v) {
        chronometer.setBase(SystemClock.elapsedRealtime());
        pauseOffset = 0;
    }
}