private var startTime;
private var restSeconds : int;
private var roundedRestSeconds : int;
private var displaySeconds : int;
private var displayMinutes : int;
var countDownSeconds : int;
function Awake() {
    startTime = Time.time;
}
function OnGUI () {
    //make sure that your time is based on when this script was first called
    //instead of when your game started
    var guiTime = Time.time - startTime;
    restSeconds = countDownSeconds - (guiTime);
    //display messages or whatever here -->do stuff based on your timer
    if (restSeconds == 60) {
        print ("One Minute Left");
    }
    if (restSeconds == 0) {
        print ("Time is Over");
        //do stuff here
    }
    //display the timer
    roundedRestSeconds = Mathf.CeilToInt(restSeconds);
    displaySeconds = roundedRestSeconds % 60;
    displayMinutes = roundedRestSeconds / 60; 
    text = String.Format ("{0:00}:{1:00}", displayMinutes, displaySeconds); 
    GUI.Label (Rect (400, 25, 100, 30), text);
}