How do I make a score script based on the time the player is alive?

Hello, I currently have a scoring script that should add points the longer the player is alive. But for some reason when I play the game the score stops at 1 and dose not go any further. Can any one tell me what I’m doing wrong?
Here is my script.

using UnityEngine;
using System.Collections;

public class HUDscript : MonoBehaviour {

float playerScore = 0;

// Use this for initialization
void Start () {

	playerScore += Time.deltaTime;
}

void OnGUI () 
{

	GUI.Label (new Rect (10, 10, 100, 30), "score: " + (int)(playerScore * 100));
}

}

Change ‘Start’ to ‘Update’.