Help with function

Help with function

Hello,

In my module Deals, i have three fields:

F. Solicitada L/P (date field)
PLAZO CONTRATO (number field)
L/P F. limite produccion (date field)

I need a function, that update L/P F. limite produccion by calculating:

F. Solicitada L/P - PLAZO CONTRATO 
Example:
F. Solicitada L/P is December 31st
PLAZO CONTRATO 30 days
L/P F. limite produccion = December 31st - 30 days = December 1st 

Thank you