1. Create a dynamic TextField and aligned to the right.

2. Give it an instance name, I call it countdown_txt.

3. In the first frame of the scene open the Actions tab and paste this code:

this.addEventListener("enterFrame",startMovie);

function startMovie(e:Event) {

	var today:Date = new Date();
	var curTime = today.getTime();

	var myRetirement:Date = new Date(2050, 7, 9);// 2050 August 9 -- months are from 0 to 11

	var myRetirementTime = myRetirement.getTime();// get the time in miliseconds of myRetirement
	var timeLeft = myRetirementTime-curTime;

	var seconds = Math.floor(timeLeft/1000);
	var minutes = Math.floor(seconds/60);
	var hours = Math.floor(minutes/60);
	var days = Math.floor(hours/24);
	var years = Math.floor(days/365);

	seconds = String(seconds%60);

	if (seconds.length<2) {
		seconds = "0"+seconds;
	}
	minutes = String(minutes%60);

	if (minutes.length<2) {
		minutes = "0"+minutes;
	}
	hours = String(hours%24);

	if (hours.length<2) {
		hours = "0"+hours;
	}
	days = String(days%365);

	if (days.length<2) {
		days = "0"+days;
	}
	years = String(years);

	var count:String = years+":"+days+":"+hours+":"+minutes+":"+seconds;
	countdown_txt.text = count;
}

Example: