Hey,
I had a quick look at this extension and have some suggestions:
* Escape ALL variables in HTML you are building. You can use htmlspecialchars, the methods of MediaWikis HTML class, and in this case probably want to just construct the whole thing with XmlSelect.
* You are not fully following the style guidelines. The spacing is off and you are using underscores in var names instead of lowerCamelCase. See http://www.mediawiki.org/wiki/Manual:Coding_conventions
* Don't use the 'description' in wgExtensionCredits, use 'descriptionmsg' instead, and properly internationalize it.
Next to that it looks great, although I'm not completely sure the i18n is all in order when using the native PHP time functions :)
Cheers
-- Jeroen De Dauw http://www.bn2vs.com Don't panic. Don't be evil. --