Rotation Question...

Hi. When a button is clicked, I want an object to rotate 90 degrees and then stop...and I want that rotation to take exactly 2 seconds. When I try this bit of code, the rotation is always a few degrees off (it rotaes about 94 degrees)...why? Thanks

 var Can_Spin = false;
 function OnGUI(){
         if (GUI.Button (Rect( Camera.main.WorldToScreenPoint (transform.position).x-15,Screen.height-Camera.main.WorldToScreenPoint(transform.position).y-.5f,25, 15),"")) {
             Can_Spin=true;Invoke("Phase_1",2);
             }
 }
 
 function Phase_1(){
         Can_Spin=false;}
         
 function Update(){
         if(Can_Spin==true){transform.Rotate(Vector3.forward * Time.deltaTime*90/2);}
 }