Membership renewal prior to expiration date

Hi, Quick question — we will be purchasing S2Member Pro for a site that is just getting under development — the client has a question about what happens if a member renews his/her membership prior to the expiration date. Will the new expiration date be one year from the original expiration date — or one year from the date they actually renewed, meaning they would lose time and we’d have to manually change the expiration?
Thank you!