stupid example
Here is a stupid example to demonstrate.
<?php
date_default_timezone_set('Europe/London');
$thissecond = date("s");?>
<!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>Untitled Document</title>
</head>
<body>
<?php echo $thissecond ;?><br />
<?php echo ($thissecond <30?'less than 30':'more than 30'); ?>
</body>
</html>
the value $thissecond is set to the current second, using the php date function.
if the value is <30, less than 30 is displayed, otherwise more than 30 is displayed.
Try loading the page then pressing f5 to refresh
using this logic, you can display whatever you want depending on the current day/ hour or whatever.
for further info on php date, see function.date.php