countdown.html

<style>
  h1 {
    font-size: 55em;
    line-height: 1.25;
    text-align: center;
  }
</style>
<h1 id=time></h1>

<script>
  var t = document.getElementById("time"),
      end = /#(\d{2}):(\d{2})/.exec(window.location.hash),
      endHour = parseInt(end[1], 10),
      endMinute = parseInt(end[2], 10);

  var poll = function() {
    var d = new Date,
        remaining = d.getHours() == endHour ? 0 : 60;

    setTimeout(function() {
      t.textContent = remaining - (d.getMinutes() - endMinute);
      poll();
    }, 1000);
  }
  poll();
</script>