I got a simple version of a countdown timer here. I'm sure there are other and more elegant versions out there (e.g. see this forum thread), but they are also a bit lengthy. See for yourself.

private var startTime;
private var restSeconds : int;
private var roundedRestSeconds : int;
private var displaySeconds : int;
private var displayMinutes : int;

var countDownSeconds : int;

function Awake() {
    startTime = Time.time;
}

function OnGUI () {
    //make sure that your time is based on when this script was first called
    //instead of when your game started
    var guiTime = Time.time - startTime;

    restSeconds = countDownSeconds - (guiTime);

    //display messages or whatever here -->do stuff based on your timer
    if (restSeconds == 60) {
        print ("One Minute Left");
    }
    if (restSeconds == 0) {
        print ("Time is Over");
        //do stuff here
    }

    //display the timer
    roundedRestSeconds = Mathf.CeilToInt(restSeconds);
    displaySeconds = roundedRestSeconds % 60;
    displayMinutes = roundedRestSeconds / 60; 

    text = String.Format ("{0:00}:{1:00}", displayMinutes, displaySeconds); 
    GUI.Label (Rect (400, 25, 100, 30), text);
}

there are many ways to do this. for example another way would be

var showText=false;

function StopWatch (time:float)
{
   yield WaitForSeconds (time);
   showText=true;
}
function OnGUI ()
{
   if (showText==true)
   {
      GUI.Label (Rect(100,100,300,150),"this text will be shown after the timer ends"):
   }
}

so how did you sort the rounding problem?? nevermind i got it all you need to do is change some variables. if anyone needs this in c# here you go :) you will need to change public CountDownSeconds in the inspector to any value of your choice ie, 10:00, you would write 600 in the inspector panel.

`using UnityEngine; using System.Collections;

public class TimerScript : MonoBehaviour {
private float startTime;
private float restSeconds;
private int roundedRestSeconds;
private float displaySeconds;
private float displayMinutes;
public int CountDownSeconds=120;
private float Timeleft;
string timetext;


// Use this for initialization

void Start () 
{
    startTime=Time.deltaTime;

}

void OnGUI()
{

    Timeleft= Time.time-startTime;

    restSeconds = CountDownSeconds-(Timeleft);

roundedRestSeconds=Mathf.CeilToInt(restSeconds);
displaySeconds = roundedRestSeconds % 60;
displayMinutes = (roundedRestSeconds / 60)%60;

timetext = (displayMinutes.ToString()+":");
if (displaySeconds > 9)
{
    timetext = timetext + displaySeconds.ToString();
}
else 
{
    timetext = timetext + "0" + displaySeconds.ToString();
}
GUI.Label(new Rect(650.0f, 0.0f, 100.0f, 75.0f), timetext);
    }}`