Need help creating Countdown Timer Stop

Here is the code that I have so far, I need to create a stop for the timer so that it stops when the timer reaches 00:00. As it stands, the timer hits 00:00 then goes straight to 0-1:59. I'm lost on how to get it to stop,  any help or feedback is greatly appreciated.

package

{

  import flash.display.Sprite;

  import flash.events.Event;

  import flash.events.TimerEvent;

  import flash.text.TextFormat;

  import flash.text.TextField;

  import flash.utils.Timer;

  import flash.text.TextFormatAlign;

public class Main extends Sprite

  {

  public var timer:Timer = new Timer(1000);

  public var txtClock:TextField = new TextField();

  public var min:int = 0;

  public var sec:int = 05;

  public var txtFormat:TextFormat = new TextFormat();

  public function Main()

  {

  if (stage) init();

  else addEventListener(Event.ADDED_TO_STAGE, init);

  timer.start();

  timer.addEventListener(TimerEvent.TIMER, changeClock);

  txtClock.x = 100;

  txtClock.y = 20;

  addChild(txtClock);

txtClock.defaultTextFormat = txtFormat;

  txtFormat.bold = true;

  txtFormat.size = 18;

  txtFormat.font = "Arial Black";

  txtFormat.color = 0xFF0000;

  txtFormat.align = TextFormatAlign.CENTER;

  }

  private function init(e:Event = null):void

  {

  removeEventListener(Event.ADDED_TO_STAGE, init);

  }

  private function changeClock(e:TimerEvent):void

  {

  var formatSec:String;

  var formatMin:String;

  sec -= 1;

  if (sec == 0)

  {

  min -= 1;

  sec = 59;

  }

  if (sec < 10)

  {

  formatSec = "0" + String(sec);

  }

  else

  {

  formatSec = String(sec);

  }

  if (min < 10)

  {

  formatMin = "0" + String(min);

  }

  else

  {

  formatMin = String(min);

  }

txtClock.text = String(formatMin + " : " + formatSec);

  txtClock.border = true;

  txtClock.width = 200;

  txtClock.height = 30;

  txtClock.borderColor = 0x0000FF;

  txtClock.setTextFormat(txtFormat);

  }