If the timeformat option is '%c', every time websetup rewrites the setup file, it changes to a ddate-style time format (but if ddate is not actually enabled, some of the format codes aren't understood by strftime, so they get passed through).
Presumably this is because websetup loads all plugins, so IkiWiki::plugin::ddate::checkconfig is run...
(This bug seems oddly appropriate. Hail Eris)