De code:
<script type="Javascript">
function afteller(x){
 span = document.getElementById('afteller');
 x = (!x) ? 30 : x;
        
 if(x > 0){
  span.innerHTML = x-1;
  setTimeout("afteller('" + (x-1) +"')", 1000);
 }
}
</script>
<span id="afteller">__HIER HET AANTAL SECONDEN DAT JE WIL AFTELLEN__</span>