Код:
<html>
<body>
<script>
<!--
var current= new Date()
var day_night=current.getHours()
if (day_night>=22){document.write("<table background='night.gif'>");}
if (day_night<=7){document.write("<table background='day.gif'>");}
//-->
</script>
<tr><td>text....</td></tr>
</table>
</body>
</html>