I have a file with an mtime of 0312 BST, or 0212 GMT. If the atom/rss feed is generated while the system timezone is set to GMT, it's correctly shown as 0212 GMT. If the system time is set to BST, it's incorrectly shown as having been created/modified at 0312 GMT.

This occurs in both RSS and Atom feeds, but not in the HTML pages (which suggests it's not the server at fault, or IkiWiki overall, but rather whatever's generating the RSS and Atom is converting timezones wrongly).

done, I think (description was not entirely clear) --Joey