Timer in Unity

I want to create a GUIText timer in Unity that displays minutes and seconds since the beginning of the level. I’ve tried using Time.deltaTime and Time.realtimeSinceStartup, but I could not get them to work. How would I script a timer in C#? Here’s where I got to:

public GUIText Timer;

void Start()
{
    startTime = Time.time;
}

void Update()
{
    newTime = Time.realtimeSinceStartup;
    int absTime = Mathf.Abs(newTime);
    int sec = absTime % 60;
    int min = absTime / 60;
    Timer.text = min + ":" + sec;
}

using UnityEngine;
using System.Collections;

public class timer : MonoBehaviour {

	// Use this for initialization
	public GUIText timerText;
	float seconds = 0f;
	float minutes = 0f;
	float newSeconds = 0f;
	void Start () {
		newSeconds = Time.realtimeSinceStartup;
	}
	
	// Update is called once per frame
	void Update () {

		seconds = Time.realtimeSinceStartup - newSeconds;
		if(seconds > 60)
		{
			seconds = 0;
			newSeconds = Time.realtimeSinceStartup;
			minutes ++;
		}


		timerText.text = "Time : "+(int)minutes +  " : " +(int) seconds;

	}
}