<script type="text/javascript">
  var count = new  Number();
  var count = 300;

  function start(){
    if ((count - 1) >=0) {
        count=count - 1;
        if (count == 0) {
          count= "Fim!";
        }
        tempo.innerText=count;
        setTimeout('start()',1000);
    }
  }
</script>