u can use add _months function if u know that its going to expire within a month..or if u want a specific date then do..
i am new try it may help u

select to_date('07-jul-99','dd-mon-rr') - hire_date as "no of days"
from employees where
to_date('07-jul-99','dd-mon-rr') - hire_date between 0 and 30

so these records wil be the ones that are going to expire