expiration attribute as an offset?

Hey guys, Is it possible to set the expiration attribute to be of an offset type? Instead of entering an absolute date is it possible to enter the duration of which the user will be allowed from the first time that he logins? Say I set an offset of +30 days, and the user logins for the first time only 6 months from now. So once he login'ed once then the counter will start counting 30 days from his first login and then expire the account. Thanks.