How do you get a countdown timer to actually count down? (C# Script)

 I'm pretty new to coding as I am studying it while in education, I found a code on how to create a countdown timer, but when I apply it to a canvas, no timer shows up. However, if i put the code into the Main Camera, the timer shows up but does not count down.

This is the code I am using.

int timer = 0;

void OnGUI(){ int minutes = Mathf.FloorToInt (timer / 60F); int seconds = Mathf.FloorToInt (timer - minutes * 60); string niceTime = string.Format ("{0:0}:{1:00}", 2, 30);

     GUI.Label (new Rect (10, 10, 250, 100), niceTime);

}

I am thankful for any help!