How can I add a timer that will start when the game starts and stop when the game ends and records it?

How can I add a timer where when the player starts the game, the timer starts and when the player finishes the game, the timer stops? Then it records it in the statistics page?

Here's a timer code: #pragma strict

 private var startTime : float;
 var textTime : String;
  
 
 function OnGUI () {
 
     var guiTime = Time.time - startTime; 
 
     var hours : int = guiTime / 3600;
     var minutes : int = guiTime / 60; 
     var seconds : int = guiTime % 60;
     var fraction : int = (guiTime * 100) % 100;
     GUI.contentColor = Color.black;
     GUI.backgroundColor = Color.black;
  
     textTime = String.Format ("{0:00}:{1:00}:{2:00}:{3:00}", hours, minutes, seconds, fraction); 
     GetComponent(GUIText).text = textTime;
  
 }

I want it to go under the "Last Playthrough Time = " in this code:

 function OnGUI() {
 
 //Font Size and Color Change
     var buttonStyle = new GUIStyle("Button");
         buttonStyle.fontSize = 17.5;
     var statsStyle = new GUIStyle("Label");
         statsStyle.fontSize = 30;
     var otherStyle = new GUIStyle("Label");
         otherStyle.fontSize = 30;
     var ostStyle = new GUIStyle("Label");
         ostStyle.fontSize = 20;
     GUI.contentColor = Color.black;
     GUI.backgroundColor = Color.white;
 
     //Statistics Title GUI Design
         GUI.contentColor = Color.white;
         GUI.backgroundColor = Color.green;
         GUI.Label(Rect(450,125,250,100), "Statistics",statsStyle);
         GUI.Label(Rect(477,160,250,100), "Other",ostStyle);
         
     //Other GUI Designs
         GUI.Label(Rect(175,250,300,300), "Total Time Played = ",otherStyle);
         GUI.Label(Rect(175,305,350,300), "Last Playthrough Time = ",otherStyle);
                 
 if(GUI.Button(Rect(725,125,150,30), "Previous Page",buttonStyle)) {
     Application.LoadLevel("Statistics2");
 }
 if(GUI.Button(Rect(125,125,150,30), "Back to Menu",buttonStyle)) {
     Application.LoadLevel("Menu");
 }
     }