If you’re having problems with Gallery2’s sitemap module, and Google Sitemaps is complaining about the time format, try the following. Go to the Gallery2modulessitemap directory and open Sitemap.inc. Change the following:
substr_replace(
gmstrftime("%Y-%m-%dT%H:%M:%S%z", $entity->getModificationTimestamp()),
":", -2, 0));
to:
gmstrftime("%Y-%m-%dT%H:%M:%S+00:00", $entity->getModificationTimestamp()));
It will hardcode the timezone offset to 0 hours, but at least your sitemap will work.