If I understand you, your case is one of taking an event defined with a date/time start and end and adding an occurrence that is defined merely by a date?
This seems like it is inconsistent with the type of event (i.e., date/time based interval). I would guess that this is an error condition.
However, I would hope that a recipient would be more forgiving and convert the date-only RDATE to a date/time where the time is from T000000 to T235959.
-- Frank