How to create a count up timer...

This is the code I have for a count down timer in days, hours, minutes, and seconds but I would just like a timer that counts up from 0 until what ever number I would like and I would like to be able to control the pace....any ideas?

var targetDate:Date = new Date(2012,3,5,13);

addEventListener(Event.ENTER_FRAME, loop);

function loop(e:Event):void

{

    var nowDate:Date = new Date();

    var ms:Number = targetDate.getTime() - nowDate.getTime();

    var sec:Number = Math.floor(ms/1000);

    var min:Number = Math.floor(sec/60);

    var hr:Number = Math.floor(min/60);

    var day:Number = Math.floor(hr/24);

    sec = sec % 60;

    min = min % 60;

    hr = hr % 24;

    daytxt.text = day.toString();

    hrtxt.text = (hr < 10) ? "0" + hr.toString() : hr.toString();

    mintxt.text = (min < 10) ? "0" + min.toString() : min.toString();

    sectxt.text = (sec < 10) ? "0" + sec.toString() : sec.toString();