use Time.timescale = 0 with time.deltatime problem

Hi , i need countdown for start game.Time.timescale is “0” when the game start.Therefore my countdown not working.

Could you help me ?

public bool isGameStart=false;
private float gameStartTime=3.0f;
public Text timerText;

void Update(){
GameStart();
}

private void GameStart(){
			
			gameStartTime -= Time.deltaTime;
			timerText.text = gameStartTime.ToString ("0");

			if (!isGameStart) {

				Time.timeScale = 0;
			} else
				Time.timeScale = 1f;
			
			if (gameStartTime <= 0) {
				isGameStart = false;
			}

		}

Time.unscaledDeltaTime is unaffected by timeScale, you can use that instead.