Did I miss something? If I am right, I offer to write the necessary patch, copied and adapted from the tag plugin, to generate the pages `archive_base/year/month.mdwn` on the fly.
--- Spalax
+-- [[Louis|spalax]]
+
+> Good spotting, `ikiwiki-calendar` predates the `add_autofile` API used to
+> autocreate tag pages and was bolted in as an easy way to create calendar
+> pages.
+>
+> It would be possible to do that inside the caneldar plugin now. Although
+> some command would still need to be run on a daily (or weekly, or
+> monthly, or yearly..) basis to have it wake up and make the new calendar
+> pages and update the displayed current day from calendar directives.
+>
+> That last is, arguably, the real point of running ikiwiki-calendar in
+> a cron job. Of course all it really does is run `ikiwiki -setup foo
+> -refresh`. --[[Joey]]
+
+> > [[Patch|todo/calendar_autocreate]]. -- [[Louis|spalax]]