function countdown();

countDown.js

function countDown() {
    var startDateTime = new Date();
    var endDateTime = new Date("March 9,2015 18:00:00");
    var left = endDateTime - startDateTime;
    var a_day = 24 * 60 * 60 * 1000;

    var h = Math.floor((left % a_day) / (60 * 60 * 1000)),
        m = Math.floor((left % a_day) / (60 * 1000)) % 60,
        s = Math.floor((left % a_day) / 1000) % 60 % 60;

    document.getElementById("#timer")[0].innerHTML = addZero(h) + ':' + addZero(m) + ':' + addZero(s);
    setTimeout('countDown()', 1000);
}
function addZero(num) {
    num = '00' + num;
    str = num.substring( num.length - 2, num.length );
    return str ;
}