This should help ;) Without any magic numbers.
var myTimer:Timer = new Timer(1000,60); // every second for 60 seconds
myTimer.addEventListener(TimerEvent.TIMER, onTimer);
myTimer.addEventListener(TimerEvent.TIMER_COMPLETE, onComplete);
myTimer.start();
function onTimer(e: TimerEvent):void {
myText_txt.text = String(myTimer.repeatCount - myTimer.currentCount);
}
function onComplete(e: TimerEvent):void{
gotoAndStop(2);
}