Having problems with my timer

Hey i need help i have made a timer and it works but i want it to activate when its in a level not from the very beginning of the game cos when its a 3 mins timer but when i built the game and went though to the level though my menu it was 2:30mins cos it was playing though the time i was on the menu please help

here is my script

var startTime : float; 
var timeRemaining : float; 

function Start()
{ 
    startTime = 180.0;
    guiText.material.color = Color.red; 
}

function Update () { 
    Countdown(); 
}

function Countdown(){ 
    timeRemaining = startTime - Time.time;
    if(timeRemaining > 0){ 
        ShowTime(); 
    }
    ShowTime(); 
    if(timeRemaining < 0) {
        timeRemaining = 0;
        TimeIsUp();
        Debug.Log("Time remaining = " + timeRemaining); 
    } 
} 

function ShowTime(){ 
    var minutes: int; 
    var seconds: int; 
    var timeString : String;
    minutes = timeRemaining / 60; 
    seconds = timeRemaining % 60; 
    timeString = minutes.ToString() + ":" + seconds.ToString("D2");
    guiText.text = timeString; 
}

function TimeIsUp(){
    Debug.Log("Time Is Up"); 
}

Thanks

could’nt you add the script to an empty gameobject in the scene, so it will only start when the scene is loaded?