codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
onload = function updateHrImg () {
    document.getElementById("clock-img").src = (new Date()).getHours() + ".gif";
    setTimeout(updateHrImg, 36e5 - Date.now() % 36e5);
};
</script>
</head>

<body>

<img id="clock-img" width="150" height="150" />

</body>
</html>