How to calculate 15th day based on a date field?

Hi,
I need to calculate a date for every cell in a Excel sheet.
Something like if I get income this month I need to pay a bill on 15th day next month!
How to automatically calculate date 15th on next month if I get several incomes on days like 23/02/2012, 17/02/2012, etc. ?
I appreciate any coments.
Thanks