Two timers out of sync

I made a test project which you can clone on Github, and documented my issue here.

In essence the problem is that I have two timers; one that counts down from 70 seconds to 0 (and then starts all over again), and the second one just shows the current client time.

Problems which I have are the following:

1. If you open up any other window you will notice that the timer will stop, and continue counting once you return to the Unity window

2. TimerText and TimeText are not “ticking off” at the same time

If someone has hints on how to approach solving this, I would be very grateful.

For reference, my code is this:

 #pragma strict
 
 var timer: float = 70;
 var isFinishedLevel : boolean = false;
 public var displayText : UnityEngine.UI.Text;
 public var timeText : UnityEngine.UI.Text;
 
 private var oldTimer : float;
 
 function Start(){
     oldTimer = timer;
 }
 
 function Update()
 {
     if (!isFinishedLevel) {
         timer -= Time.deltaTime;
     } 
 
     if (timer > 0) {
         var minsDisplay : String = parseInt( timer / 60 ).ToString();
         var secsDisplay : String = parseInt( timer ).ToString();
          
         if ( (timer - ( parseInt(minsDisplay) * 60)) > 10 ) {
              secsDisplay = parseInt( timer - ( parseInt(minsDisplay) * 60) ).ToString();
         } 
         else {
             secsDisplay = "0" + parseInt( timer - ( parseInt(minsDisplay) * 60) ).ToString();
         }
         
         displayText.text = minsDisplay + " : " + secsDisplay;
     } 
     else {
          timer = oldTimer;
     }
     
     CurrentTime();
 }
 
 function CurrentTime() { 
     var dt : System.DateTime = System.DateTime.Now;
     var h : int = dt.Hour; 
     var m : int = dt.Minute; 
     var s : int = dt.Second;
 
     timeText.text = h + ":" + m + ":" + s;
 }