Create Count Down Timer in Javascript

In this simple code you can create count down timer in just few step .


<span id="countdown" class="timer"></span>
<script>
         var seconds = 20;  // change time in second
                function secondPassed() {
         var minutes = Math.round((seconds - 30)/60);
         var remainingSeconds = seconds % 60;
        if (remainingSeconds < 10) {
                 remainingSeconds = "0" + remainingSeconds;
         }
 
       document.getElementById('countdown').innerHTML = minutes + ":" +    remainingSeconds;
       if (seconds == 0) {
                clearInterval(countdownTimer);
                          document.getElementById('countdown').innerHTML = "Time is over";
        } else {   
                seconds--;
       }
    }
var countdownTimer = setInterval('secondPassed()', 1000);
</script>