如何在ActionScript 2中检测鼠标离开舞台的时间？

var lastTime:Number;

function onMouseOut(event:MouseEvent):Void {
    lastTime = getTimer();
}

function onMouseOver(event:MouseEvent):Void {
    var currentTime:Number = getTimer();
    var timeAway:Number = currentTime - lastTime;
    trace("鼠标离开舞台的时间为：" + timeAway + "毫秒");
}

stage.addEventListener(MouseEvent.MOUSE_OUT, onMouseOut);
stage.addEventListener(MouseEvent.MOUSE_OVER, onMouseOver);