import flash.text.TextField;

var timeTextfield:TextField = myTextFieldOnStage;
timeTextfield.text = getFormattedTime();

var clockTimer:Timer = new Timer(1000, 0);
clockTimer.addEventListener(TimerEvent.TIMER, onClockTimer);
clockTimer.start();

function onClockTimer(e:TimerEvent):void {
	timeTextfield.text = getFormattedTime();
}

function getFormattedTime():String {
	var now:Date = new Date();
	var hrs:String = String(now.getHours());
	if (hrs.length < 2) {
		hrs = "0" + hrs;
	}
	var mins:String = String(now.getMinutes());
	if (mins.length < 2) {
		mins = "0" + mins;
	}
	return hrs + ":" + mins;
}

This example expects there to be a dynamic textfield on the stage with the instance name of 'myTextFieldOnStage'. Remember to embed the fonts (Numerals and Colon).

AS3 Simple digital clock

format

ActionScript 3