For most of the events we use on our site the hour is irrelevant. They'd better
fit as All day events.

Mozilla just saves the date (no hours and minutes), I don't know how about MS,
and there is a option for all day events in Funambol (X-FUNAMBOL-ALLDAY).

It seems to me as a good idea to provide the users with checkbox that would
"hibernate" the start and end time fields.