Get the difference between two Calendar

Modify the exercise "android.app.TimePickerDialog", calculate the difference between time and next set time in TimePickerDialog.

package com.exercise.AndroidTime;

import java.util.Calendar;

import android.app.Activity;
import android.app.TimePickerDialog;
import android.app.TimePickerDialog.OnTimeSetListener;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;
import android.widget.TimePicker;

public class AndroidTimeActivity extends Activity {
 
 TimePicker myTimePicker;
 Button buttonstartSetDialog;
 TextView textPrompt;
 
 TimePickerDialog timePickerDialog;
 
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        textPrompt = (TextView)findViewById(R.id.prompt);
        
        buttonstartSetDialog = (Button)findViewById(R.id.startSetDialog);
        buttonstartSetDialog.setOnClickListener(new OnClickListener(){

   @Override
   public void onClick(View v) {
    openTimePickerDialog(false);
    
   }});

    }

  
 private void openTimePickerDialog(boolean is24r){
  Calendar calendar = Calendar.getInstance();
  
  timePickerDialog = new TimePickerDialog(
    AndroidTimeActivity.this, 
    onTimeSetListener, 
    calendar.get(Calendar.HOUR_OF_DAY), 
    calendar.get(Calendar.MINUTE), 
    is24r);
  timePickerDialog.setTitle("TimePickerDialog Title");  
        timePickerDialog.setMessage("TimePickerDialog Message"); 
        
  timePickerDialog.show();

 }
    
    OnTimeSetListener onTimeSetListener
    = new OnTimeSetListener(){

  @Override
  public void onTimeSet(TimePicker view, int hourOfDay, int minute) {

   Calendar calNow = Calendar.getInstance();
   Calendar calSet = (Calendar) calNow.clone();

   calSet.set(Calendar.HOUR_OF_DAY, hourOfDay);
   calSet.set(Calendar.MINUTE, minute);
   
   String stringPrompt = "Current Time is: " + calNow.getTime() + " / " + calNow.getTimeInMillis() + "\n"
     + "Set Time is: " + calSet.getTime() + " / " + calSet.getTimeInMillis() + "\n"
     + "calSet.compareTo(calNow) = " + calSet.compareTo(calNow) + "\n\n";
   
   if(calSet.compareTo(calNow) > 0){
    //Today Set time not yet passed
    long diff = calSet.getTimeInMillis() - calNow.getTimeInMillis();
    stringPrompt += "Millis to set time = " + diff;
   }else{
    //Today Set time passed, count to tomorrow
    calSet.add(Calendar.DATE, 1);
    long diff = calSet.getTimeInMillis() - calNow.getTimeInMillis();
    stringPrompt += "Millis to tomorrow set time = " + diff;
   }
   
   textPrompt.setText(stringPrompt);
  }};

}

<?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" >

    <TextView
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello" />
    <Button
        android:id="@+id/startSetDialog"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" 
        android:text="Set Target Time"/>
    <TextView
        android:id="@+id/prompt"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" />
</LinearLayout>