Horas em tempo real

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Horas em tempo real com javascript!</title>
<script>
function showTime(){
    var time = new Date();
    var hour = time.getHours();
    var minute = time.getMinutes();
    var second = time.getSeconds();

    if(hour<10) hour = "0"+hour;
    if(minute<10) hour = "0"+minute;
    if(second<10) hour = "0"+second;
    var tempo = hour+":"+minute+":"+second;

    document.getElementById("timer").innerHTML=tempo;
}
function initTime(){
    setInterval(showTime, 1000);
}
</script>
</head>

<body onLoad="initTime();">
    <span id="timer">Horas</span>
</body>
</html>