Thanks for your input.
This is not a bug, but a warning message only. It means that your php settings do not allow to change the maximum execution time of the script, which is not default PHP setting and is applied usually in some shared hosting environments. This doesn't mean you can't use Sitemap generator - it will still crawl the site and create the sitemap, the only problem is that for large sites (with a lot of pages) it may be not enough time to complete it (that's why the script tries to extend the limit). You should contact your hosting support regarding this if this will be the case.