Unity - Dem nguoc thoi gian

// file: CountdownTimer.cs
using UnityEngine;
using System.Collections;

public class CountdownTimer : MonoBehaviour {
    private float secondsLeft = 10f;

    private void OnGUI(){
        if( secondsLeft > 0)
            GUILayout.Label("Countdown seconds remaining = " + (int)secondsLeft     );
        else
            GUILayout.Label("Countdown has finished");

    }

    private void Update(){
        secondsLeft -= Time.deltaTime;
    }
}