Well, today is populated by a javascript include (IIRC), so if these pages are PHP you should insert this into your header on any pages which have the mini calendar:
<script language="javascript" type="text/javascript">
var baseurl = "/forums";
var fullurl = "https://www.ubbcentral.com/forums";
var today = '<?php echo(date("d")); ?>';
</script>
Be sure to replace lines 2 and 3 with your board specific information.
