You just need to have a variable that increments itself each second. Here's a very simple script. First go to GameObject>CreateOther>GUIText.

Drag this script into an empty game object and add the GUIText that you created into the timeDisplay variable in the inspector window

var playedTime : float;
var timeDisplay : GUIText;

function Start(){
    playedTime = 0.0;
}    

function Update(){

    playedTime += Time.deltaTime;
    timeDisplay.text = Mathf.RoundToInt(playedTime).ToString();

}

This script should work, tried already. I Hope this helped

-Vatsal