Countdown Timer

Hi....

I'm having trouble with my timer.

I can't seem to get it to start at zero.

This is my script:

 #pragma strict
 
 var timer : float = 10.0;
 
 function Update()
 {
     
     timer = -= Time.deltaTime;
 }
 function OnGUI()
 {
     GUI.Box(new Rect(10,10,60,20), "" + timer.ToString("0.0"));
 
 }

If you can help, Please do...

Thanks!