ActionScript 3.0 :: Rotate MovieClip With Certain Speed

I'm lost and new to AS3. How to get this code to ease into end point compared to its sudden stop.

CODE:
addEventListener(Event.ENTER_FRAME, mcRotation);
var rotateSpeed:Number = 0.003
var oldvalue:Number = 0;
function mcRotation(event:Event):void{
var controller:Number = (stage.mouseX - stage.stageWidth * 0.5) * -1;
var rotY:Number = box_mc.rotationY - (controller * rotateSpeed);
rotY = Math.max(rotY, -90);
rotY = Math.min(rotY, 0);
box_mc.rotationY = rotY;
oldvalue = box_mc.rotationY;
}