How can I make a Day countdown timer in flash

There are numerous syntax errors in your code, especially usage of Date methods - getFullYear, getMonth, etc. To begin with, this code should look like this:

import flash.events.Event;
// register function
addEventListener(Event.ENTER_FRAME, daytimer_handler);
// calls repeatedly
function daytimer_handler(e:Event):void
{
// current date
          var today:Date = new Date(2013, 4, 22);
// current Year
          var currentYear = today.getFullYear();
// current month
          var currentMonth = today.getMonth();
// current day
          var currentDay = today.getDate(); // current time
          var currentTime = today.getTime();
// target date (5 days from now change to your need
          var targetDate:Date = new Date(currentYear, currentMonth, currentDay + 5);
          var targetDay = targetDate.getTime();
// time remaining
          var timeLeft = targetDay - currentTime;
          var sec = Math.floor(timeLeft / 1000);
          var min = Math.floor(sec / 60);
          var hours = Math.floor(min / 60);
          var days = Math.floor(hours / 24);
  
// convert sec to string
          sec = String(sec % 60);
  
// if less than add a 0
          if (sec.length < 2)
          {
                    sec = "0" + sec;
          }
  
          min = String(min % 60);
          if (min.length < 2)
          {
                    min = "0" + min;
          }
          hours = String(hours % 24);
          if (hours.length < 2)
          {
                    hours = "0" + hours;
          }
          days = String(days);
          if (timeLeft > 0)
          {
// display day string
                    var dayCounter:String = days;
                    timer_display.text = dayCounter;
          }
          else
          {
                    trace("Happy Birthday!");
                    var newTime:String = "0";
                    timer_display.text = newTime;
                    removeEventListener('enterFrame', daytimer_handler);
          }
}

to be sure, that's a syntax error

if today is a date instance, use:

today.getTime();