Want to be able to start with them blank.
Have sorting work (would if months always a zero, but they come out with and without).
Be able to add a time.
Express beginning/ending date/time
Recurring dates (Google Calendar is a good peer here)
Sometimes you're not sure yet when something is happening. Express fuzzy dates (maybe a Pointer of dates?)
Integrate with or better yet just implement our own group date-finding tools.
(Also see Implement microformats - so that we can use http://microformats.org/wiki/hcalendar hcalendar.)