android:format;//定义时间的格式如：hh:mm:ss
 setFormat("计时:%s"); //设置显示格式
 setFormat(String format);//设置显示时间的格式。 
start();//开始计时 
stop();//停止计时

setBase（）;//设置基地时间，一般都是SystemClock.elapsedRealtime() 
setOnChronometerTickListener(Chronometer.OnChronometerTickListener listener);//当计时器改变时调用

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:orientation="vertical" >
  <Chronometer
    android:id="@+id/chronometer"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
   />
  <Button
    android:id="@+id/chronometer_start"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="开始计时"
  />
  <Button
    android:id="@+id/chronometer_end"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="停止计时"
     
  />
  <Button
    android:id="@+id/chronometer_null"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="重新开始计时"
     
  />
</LinearLayout>

package com.test;
 
import android.app.Activity;
import android.os.Bundle;
import android.os.SystemClock;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Chronometer;
 
public class ChronometerDemo extends Activity {
   
  private Chronometer chronometer;
   
  private Button button_start, button_end,button_bull;
   
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
     
    setContentView(R.layout.chronometer);
     
    chronometer =(Chronometer)findViewById(R.id.chronometer);
    button_start =(Button)findViewById(R.id.chronometer_start);
    button_end =(Button)findViewById(R.id.chronometer_end);
    button_bull =(Button)findViewById(R.id.chronometer_null);
     
    button_start.setOnClickListener(clickListener);
    button_end.setOnClickListener(clickListener);
    button_bull.setOnClickListener(clickListener);
  }
 
  private OnClickListener clickListener = new OnClickListener() {
     
    @Override
    public void onClick(View v) {
       
      switch (v.getId()) {
      case R.id.chronometer_start:
        //调用start()方法开始计时
        chronometer.start();
        button_start.setText("正在计时...");
        break;
      case R.id.chronometer_end:
        //调用stop()方法停止计时
        chronometer.stop();
        button_start.setText("继续计时");
        break;
      case R.id.chronometer_null:
        //调用stop()方法停止计时
        chronometer.setBase(SystemClock.elapsedRealtime());
        chronometer.start();
        button_start.setText("正在计时...");
        break;
 
      default:
        break;
      }
    }
  };
}