Timer Count backward?

I get this one from http://answers.unity.com/answers/1088328/view.html
works really well, but I wondering how can I count backward?

using System;

DateTime startTime;
TimeSpan elapsedTime;

void Start()
{
    startTime = DateTime.Now;
}

void LateUpdate()
{
    elapsedTime = DateTime.Now - startTime;

    string displayTime = String.Format("{0:60}:{1:00}", elapsedTime.Minutes, elapsedTime.Seconds);

    GetComponent<TextMesh>().text = displayTime;
}

Following code not tested:

 DateTime endTime;
 TimeSpan remainingTime;
 void Start()
 {
     endTime = DateTime.Now.AddSeconds( 60 ); // for instance
 }
 void LateUpdate()
 {
     remainingTime =  endTime - DateTime.Now;
     if( remainingTime.Minutes >= 0 && remaining time.Seconds >= 0 )
     {
         string displayTime = String.Format("{0:60}:{1:00}", remainingTime.Minutes, remainingTime.Seconds);
         GetComponent<TextMesh>().text = displayTime;
     }
 }