Add the following in:
<meta HTTP-EQUIV="REFRESH" CONTENT="1">Where CONTENT corresponds to the refresh interval, here is 1 second.
Full example (test.html):
<!DOCTYPE html>
<html>
<head>
<meta HTTP-EQUIV="REFRESH" CONTENT="1">
<title>zifeiy's auto refresh test</title>
</head>
<body>
<span id="showtime"></span>
</body>
<script language="javascript">
document.getElementById("showtime").innerHTML = "" + new Date();
</script>
</html>