Turn seconds into minutes and seconds MM:SS

I have this code with a timer on, this timer is displayed on one of my scenes. Now it is working as supposed, there is just a little thing, the timer is only showing in seconds. How would i modify my code, so that it is showing both minutes and seconds like MM:SS, while the rest of the code is still working?

 static var energy : int = 5;
 
 var timer : float;
 
 function Start()
 {
 timer = 1800;
 }
 
 function Update () {
  
   timer -= Time.deltaTime;
      if(counter <= 0){
         energy += 1;
         counter = 1800;
         Debug.Log("You Have " + energy + " Energy"); 
 }
 
 function OnGUI () {
 
     GUI.Label(new Rect(100,20,400,40),"To Next: " + timer);
 }