"How to make and display in GUI timer"

I have similar problem to how to [Start a Timer] & Display as GUI - Unity Answers in my racer game. I’m looking step by step tutorial “How to make and display in GUI timer”. I have 2 line: starting line and finisz line. If player cross starting line - time start. But if player cross finish line - time stop.

I tried add script from above link to starting object and script doesn’t work.

My car’s inspector look like:

13631-inspector_car.png

My cube’s inspector look like:

13632-inspector_cube.png

My code for timer attach to cube:

using UnityEngine;
using System.Collections;
 
public class Timer: MonoBehaviour 
{
  private float startTime;
  private float elapsedTime;
 
  void Awake(){
 startTime = 0;      
  }
 
  void Update () {
 
  if (startTime > 0)
    {
       elapsedTime = Time.time - startTime;
    }
  }

  void OnTriggerEnter(){
    startTime = Time.time;
  }
 
void OnTriggerExit(){
    startTime = 0;
  }
 
  void OnGUI(){
 GUI.Label(new Rect(300, 100, 100, 20), (elapsedTime.ToString()));
  }
}

If I go near to cube - timer doesn’t count time and display 0.