I recall the post of Dave_L regarding the merging of the two fields. At the moment there are two fields one for the date and one for the time, however I don't remember if the time is used. The null value on the time that I was thinking could in effect set the time to be 00:00:00 but also set a new field "AllDayEvent" to yes. this way we can cater for events that will occur at midnight but also get rid of the time if we have an all-day event.
There are also some other fields there for recurrance etc. Those need additional consideration in the future.