Stop and Resume Timer

I have a problem with stop and resume timer. everytime i'm trying to stop the timer and then resume it again the timer has decreased too much time. Firstly > Timer Start @ 30.0 > @ 27.4 Timer Stop > Wait 1 or 2 seconds and then starts the timer again > The timer goes 25.2 not 27.4. does anyone know why or how?

this is my script.

 private var enableTimer = false;
 private var remains : float;
 var timer : float = 30.0;
 var txt : TextMesh;
 var objects : GameObject[];
 
 function Start(){
     if(txt == null) GameObject.Find("Timer").GetComponent(TextMesh);
     txt.text = "0";
     remains = timer;
 }
 
 function Update () {
     //timer -= Time.time;
     if(enableTimer){
         var clampedTime : float = Mathf.Clamp(Time.time,timer,0);
         clampedTime -=Time.time;
         if(txt) txt.text = clampedTime.ToString("f1");
     }
 }
 
 
 function OnTriggerEnter(col : Collider){
     if(col.gameObject.tag == "Player"){
         enableTimer = true;
     } else enableTimer = false;
 }