Just released a new version (2.8) of the calendar class with a bug fix for those using the class on windows servers.
The windows platform does not support the %e (day of month) modifier to the strftime function. Instead, the modifier %#d must be used to achieve the same date format. Se the notice on the strftime function description page on php.net.
View the claendar.class.php project page for download or svn access to the new version.
Also, I would like to greatly thank Steffen Maaß for bringing this bug to my attention! Sorry it took so long to get it implemented.