Assumably your creation date will always be the date that expires first.
You will need the following fields:
-creationDate - date type of field with auto-enter creation date
-expirationDate - calculation type of field, the equation for the calc is:
Month (creationDate) & "/" & Day (creationDate) & "/" & ( Year (creationDate) + numberofyearstoexpiration )
Then you will want the following script, which I would suggest to run on open of the file
-set error capture ON
-goTo records table
-enter find mode (no pause)
-set field expirationDate - "=<" & get(currentDate)
-if (get (lastError) =/= 401 )
-Show custom dialog "purge these records"
-continue to regular start of database
Hope this makes sense and puts you in the right direction!