countdown never reaches 0 and gives negative value
<p>I have a count down script, but I have 2 issues. 1. my countdown never reaches 0 and stops so i get a continous negative number. 2. the counter only displays in chrome and not firefox or internet explorer.How do I fix these two isses?</p>

<pre><code>var sec, min, hr, day, timer, expireDate;

sec = 1000;
min = 60 * 1000;
hr = 60 * 60 * 1000;
day = 24 * 60 * 60 * 1000;
timer;
expireDate = new Date('Mon Sep 17 2012 14:26:00 GMT-0700');

timer = setInterval(function() {
    var currentDate, countdown, days, hours, minutes, seconds;
    currentDate = new Date();
    countdown = expireDate - currentDate;
    if (countdown === 0 ) {
        window.clearInterval(timer); 
        document.getElementById('countdown').innerHTML = 'EXPIRED!';
    }

    days = Math.floor(countdown / day);
    hours = Math.floor((countdown % day) / hr);
    minutes = Math.floor((countdown % hr) / min);
    seconds = Math.floor((countdown % min) / sec);
    console.log(countdown);
    document.getElementById('countdown').innerHTML = days + " " + hours + " " + minutes + " " + seconds;
}, 1000);​
</code></pre>
