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));
 }

}