js倒计时

<span id="countdown_hour"></span>:
<span id="countdown_minute"></span>:
<span id="countdown_second"></span>
<script type="text/javascript">
    var nowTime = new Date().getTime();
    var endTime = new Date("Jun 18, 2011 10:00:00").getTime();
    var countDownTimer = null;
    function get2Num(a) { if (a < 10) { return "0" + a } return "" + a; }
    function countDown() {
        var _diff = parseInt((endTime - nowTime) / 1000);
        var _day = parseInt(_diff / 3600 / 24);
        var _hour = Math.floor((_diff / 3600) % 24);
        var _minute = Math.floor((_diff / 60) % 60);
        var _second = Math.floor(_diff % 60);
        _hour = _day > 0 ? (_day * 24) + _hour : _hour;
        if (_diff <= 0) {
            window.clearInterval(countDownTimer);
            updateLED(0, 0,0);
            return;
        }
        updateLED(_hour,_minute,_second);
        nowTime += 1000;
    }
    function updateLED(h, m, s) {
        document.getElementById("countdown_hour").innerHTML = get2Num(h);
        document.getElementById("countdown_minute").innerHTML = get2Num(m);
        document.getElementById("countdown_second").innerHTML = get2Num(s);
    }
    countDownTimer = window.setInterval(countDown, 1000);
</script>