Event.observe(window, 'load', function(){             
  setInterval(function(){
    var time_now = new Date();
    var minutes = time_now.getUTCMinutes()
    minutes = ( minutes < 10 ? "0" : "" ) + minutes;
    var hours = time_now.getUTCHours()
    hours = ( hours < 10 ? "0" : "" ) + hours;
    document.getElementById("time_now").innerHTML = hours + ':' + minutes;
  }, 5000); 
});