Calculated Date..?

Here's the situation.
I'm creating a database for my county, keeping track of all the rabies vaccinations that our organization does. In the form that is used to enter the vaccination data (e.g. owner's name, address, animals name, date of vaccination, etc) The user inputs the Date of Vaccination and whether it is a "1 Year" or "3 Years" vaccine and then they have to fill in another field called Vaccination Expiration, which is the year in the Date of Vaccination plus either 1 or 3 years, depending on which option is chosen. I need to know how to make the Vaccination Expiration field fill in automatically so as to reduce error in the calculation process.

Name ------------------ Description
DATEVAC Date of Vaccination
LENGTH Vaccination Length (1 Year or 3 Years)
VACEXP Vaccination Expiration Date
ANIMAL_VACCINATION Name of the Table AND Form that the data is inputed into