Runescape Membership Pre-Paid Cards. Do they overlap?

I am curious if the days will just keep going if i buy two pre-paid cards. For instance, If I buy one 90 day membership card and sign it up in May(so it expires in July), and then buy another 90 day membership card and sign it up in June, will the expiry date be moved to September? or only 90 days from the last card I signed up, so June-August? If i use 60 days of a 90-day prepaid card, and then put another 90-day card, will it be up to 120 days left?