calculation on textboxes that have date value?

i have three textboxes named borrower_date_txt,period_txt and ret_date_txt ... know i want to make calculation on those textboxes  ... i want to enter the date into the borrower_date_txt (not only today's date) and in the period_txt i enter a period for example (5) in the ret_date_txt i want to display the borrower_date_txt + 5 i.e give me the date after 5 days of 'borrower_date_txt' date value?