There was some effort to do this as detailed here.
-http://ikiwiki.info/todo/Set_arbitrary_date_to_be_used_by_calendar_plugin/
+[[http://ikiwiki.info/todo/Set_arbitrary_date_to_be_used_by_calendar_plugin/]]
I had a quick go at doing something similar on Debian Stable (Ikiwiki 3.0) but alas my Ikiwiki fu is not strong enough.