This date overrides the <lastmod> tag for older items. It is introduced to make Google re-index the page after code updates which change the URL structure, page content or sitemap.
This sitemap suggests Google to index the "details" view. The "details"-URLs are change-resistant because the detailsItem GET parameter determines which image is displayed.
This method returns a sub-sitemapmap as DOMDocument. It builds URLs for the web frontend. If a specialized method $target.'Map'() exists, this method returns it's return value. Otherwise it uses some default code for containers, i.e. groups, films and series. The sitemaps strongly depend on some code in the gallue web frontend which determines which GET parameters return which content. I am not quite sure how Google handles the URLs provided in a sitemap, but I guess that the URLs in the sitemap should match exactly those in the web frontend. Therefor you have to honor the "default value" behavior of GALGlobalVars and the exact order of variables, although the web frontend would also accept another order.
