This should get you started:

private var startTime;
var textTime : String; //added this member variable here so we can access it through other scripts


function Awake() {

   startTime = Time.time;

}

function OnGUI () {

   var guiTime = Time.time - startTime;

   var minutes : int = guiTime / 60;
   var seconds : int = guiTime % 60;
   var fraction : int = (guiTime * 100) % 100;

   text = String.Format ("{0:00}:{1:00}:{2:000}", minutes, seconds, fraction); 
   GUI.Label (Rect (400, 25, 100, 30), textTime); //changed variable name to textTime -->text is not a good variable name since it has other use already

}

In your OnTriggerEnter function you then have to write the following to access the time when the trigger was entered (adjust "Main Camera" and "Timer" to whatever your game object and script name are that contain your timer):

function OnTriggerEnter (other : Collider) {
 var timeTrigger = gameObject.Find("Main Camera").GetComponent("Timer").textTime;
 Application.LoadLevel (1);//or whatever the number of your new level is
}

Plz how to make this timer reversed ? For example from 30.00 to 0... Thanks.

you should get the time.time in a awake or start function in new scene and then subtract the current time.time from the first value. somethings like this

var t,ellapsed;
function Start ()
{
t=time.time;
}
function Update ()
{
ellapsed=time.time-t;
}

can i noe where to place all these method at in unity?