Calculate age in filemaker pro 11 not using current date but a custom date

I want to calculate "age" but i`m using a field called "birthdate" and one called "interview date". What must i do to auto calculate the field "age " when i enter firST the "interview date" (21/6/2011) and then the "birthdate" (20/3/1972).
Thanks in advance.
Please help anyone