<script type="text/javascript">
now = new Date();

if ((now.getHours() > 9) && (now.getHours() < 23))
{
    document.write("<style>body{background:url(wall1.jpg);}</style>"); // Wallpaper from 9-23
}
else
{
    document.write("<style>body{background:url(wall2.jpg);}</style>"); // Wallpaper from 23-9
}
</script>