Hi there i´m new to unity. i have built a labyrinth with timer. how can i set the timer backwards from 15sec. counting down and stop the game at 0?

Timer:

using UnityEngine; using System.Collections; using UnityEngine.UI;

public class Timer : MonoBehaviour {

 public Text counterText;

 public float seconds, minutes;


 // Use this for initialization
 void Start () {
     counterText = GetComponent <Text> () as Text;

 }

 // Update is called once per frame
 void Update () {
     minutes = (int)(Time.time / 60f);
     seconds = (int)(Time.time % 60f);
     counterText.text = minutes.ToString ("00") + ":" + seconds.ToString ("0000");

         }
         }

PlayerController:

using UnityEngine; using UnityEngine.UI; using System.Collections;

public class PlayerController : MonoBehaviour {

 public float g=9.8f;
 public Text countText;
 public Text winText;
 private Rigidbody rb;
 private int count;


 void Start ()
 {
     rb = GetComponent<Rigidbody>();
     count = 0;
     SetCountText ();
     winText.text = "";
 }

 void FixedUpdate() {
     // normalize axis
     var gravity = new Vector3 (
         Input.acceleration.x,
         Input.acceleration.z,
         Input.acceleration.y
     ) * g;

     GetComponent<Rigidbody>().AddForce (gravity, ForceMode.Acceleration);
 }

 void OnTriggerEnter(Collider other) 
 {
     if (other.gameObject.CompareTag ( "Pick Up"))
     {
         other.gameObject.SetActive (false);
         count = count + 1;
         SetCountText ();
     }
 }

 void SetCountText ()
 {
     countText.text = "Count: " + count.ToString ();
     if (count >= 12)
     {
         winText.text = "Spitze!";
     }
 }

}