Counting down for x to 0 in Javascript?

window.onload = function () {
    var countdown_start_in_ms = 6000; // from server

    function tick() {
        var now = new Date().getTime();
        var disp = start - now;

        if (disp < 0) {
            disp = 0;
        }

        var el = document.getElementById("countdown");

        el.innerHTML =
            // quick hack to format time
            /(dd:dd:dd) ...$/.exec(new Date(disp).toUTCString())[1];

        if (disp > 1000) {
            var elapsed = new Date().getTime() - now;
            window.setTimeout(tick, 1000 - elapsed);
        } else {
            // stop countdown and set color to light grey
            el.style.color = "#ccc";
        }
    }

    var start = new Date().getTime() + countdown_start_in_ms;
    tick();
}

$('#until2d4h').countdown({until: '+12M +54S'});