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");
}
	}

Sounds like you want a Stopwatch.

using System;
using System.Diagnostics;
using System.Threading;

public class Timer
{
	protected Stopwatch timer = new Stopwatch();
	protected TimeSpan time = null;
	void Start()
	{
		timer.Start();
	}
	
	void OnApplicationQuit()
	{
		timer.Stop();
		time = timer.Elapsed;
		Debug.log(time);
	}
}