Check out example codes for "unity timer". It will help you in understanding the concepts better.

Code Example 1

float timeLeft = 30.0f;
     
     void Update()
     {
         timeLeft -= Time.deltaTime;
         if(timeLeft < 0)
         {
             GameOver();
         }
     }

Code Example 2

function Update() 
 {     
	timeLeft -= Time.deltaTime;     
 	if ( timeLeft < 0 )     
 	{         
 		GameOver();     
 	} 
 }

Code Example 3

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

public class  Timer : MonoBehaviour
{
    public int timeLeft = 20;
    public Text countdownText;
    GameObject timeuptext;
    GameObject timeend;
    public int timeCountdownPlays;
    public AudioClip CountDownSound;
    public AudioSource SoundSource;
    

    
    void Start()
    {
        timeuptext = GameObject.Find("TimeUp");
        timeuptext.SetActive(false);
        timeend = GameObject.Find("Timer Text");
        StartCoroutine("LoseTime");
        SoundSource.clip = CountDownSound;
    }

  
    void Update()
    {
       
        countdownText.text = ("" + timeLeft);
            

        if (timeLeft <= 0)
        {
            timeuptext.SetActive(true);
            timeend.GetComponent<Text>().enabled = false;
            
        }

        if (timeLeft == timeCountdownPlays)
        {
            SoundSource.Play();
        }
            
            
        
      
       
    }

    IEnumerator LoseTime()
    {
        while (true)
        {
            yield return new WaitForSeconds(1);
            timeLeft--;
        }
    }
}

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