Make a Simple Count Down Timer in C# Unity3d

private float timer;
public Text txtTimer;

timer = Time.timeSinceLevelLoad;
        int minutes = Mathf.FloorToInt(timer / 60F);
        int seconds = Mathf.FloorToInt(timer - minutes * 60);
        string niceTime = string.Format("{0:0}:{1:00}", minutes, seconds);
        txtTimer.text = niceTime;