How do i make a timer show up only on certain levels?

I made a timer script, but i want it to show up only on certain levels.

#pragma strict

var timer : float = 30.0;

function Update () 

{
timer -= Time.deltaTime;

if(timer <= 0)

{
timer = 0;
Application.LoadLevel ("End");
}

}

function OnGUI ()

{

GUI.Box (new Rect (10,10,50,20), "" + timer.ToString ("0"));

}

Is there something i can add to the code for that to happen? Thank You!

Application.loadedLevel will tell you the current level. So you can check that. Or only add that script to the camera of levels that need to display the time.