Calculate new exp. date not only from today

There are two use-cases that need to be differentiated when a new expiration date is calculated

(1) Current expiration date is in the past
-> calculate from today
(2) current expiration date is in the future or today
-> calculate from given expiration date

Background:
Especially if there are annual user fees, a user who comes to renew a week before the expiration date would want to have a full new year starting from the current expiration date