JSFiddle - React, Tailwind, and code Playground

HTML

<div id="countdown"></div>

JavaScript

// countdown
var start=new Date('2012/01/20 10:00:00'),
    end=new Date('2012/01/20 10:01:00'),
    t=end-start, interval=1000/250, fnCountdown=null;

function component(x,v){
    return Math.floor(x/v);
}
function countDown(){
    t=t-interval;
    var days = component(t, 24 * 60 * 60 * 1000),
        hours = component(t, 60 * 60 * 1000) % 24,
        minutes = component(t, 60 * 1000) % 60,
        seconds = component(t, 1000) % 60,
        milliseconds = t % 1000;
    if(t<=500){
        $('#countdown').html('Over!!');
        clearInterval(fnCountdown);
    }
    else{
        if(days<10) days = '0'+days;
        if(hours<10) hours = '0'+hours;
        if(minutes<10) minutes = '0'+minutes;
        if(seconds<10) seconds = '0'+seconds;
        milliseconds = (milliseconds+'').substr(0,3);
        //if(milliseconds<10) milliseconds= '00'+milliseconds;
        //else if(milliseconds<100) milliseconds= '0'+milliseconds;
        $('#countdown').html(days + ' : ' + hours + ' : ' + minutes + ' : ' + seconds + '.' + milliseconds);
    }
}

fnCountdown=setInterval(function(){countDown();}, interval);