In theory if you add the time to Start Date and to End Date you should have all day event. This is how I did it :
Assume taht Start Date : 25/06/2011 and End Date 28/06/2011
The way I save to Database is :
Start Date (DateTime type) : 25/06/2011 08:00 ( Assume that Bussiness hours starts at 08:00)
EndDate (DateTime type) : 25/06/2011 18:00 ( Assume that Bussiness hoursend at 18:00)
So when data is fetch from database DayPilot Lite sets the event as all day event.