title: countdown never reaches 0 and gives negative value
body: <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>