Help Countdown Timer

Hi there, I want to add a millisecond field into this code but not sure how, can anyone assist?

var my_format:TextFormat = new TextFormat();
var start = getTimer();
var countdown = “60:00”;
var textbox = this.createTextField(00, 00, 250, 175, 200, 300);
textbox.variable = “countdown”;
var runit = function () { var diff = (getTimer()-start)/1000;
var mins = 60±Math.floor(diff/60);
var secs = 60-Math.floor(diff%60);
var msecs = 1000-Math.floor(diff%60);
countdown = mins+":"+secs;
if (countdown == “00:00:00”) {trace(“time’s up”);
clearInterval(timer);}
};
var timer = setInterval(runit, 1000);