Date Produced based on hours

I've got essentially three textboxes.

Texbox one = End Date
Textbox Two = Hours
Textbox Three = Start Date


I want to fill in the End Date, then fill in the amount of hours. Based on the amount of hours picked.

32 hours picked
End Date 3/8/2011
start date = 1/8/2011

So it displays start date based on amount of hours taken off the end date


End date textbox is already formatted to only accecpt dd/mm/yyyy format.
Really appreciate the help.