Making the subscriptions cover only future posts

Hiya. I'd like to arrange my subscription option such that subscribers get access only to articles that are released during the time that they are subscribers (and not posts that were released before they signed on to subscribe). This is how normal subscriptions work. Is there a way that I can arrange this?

Also, I'd like to give readers the option to subscribe for 1 year, 6 months, or 3 months. Is there a way that I can arrange for multiple subscription options? Thank you for your time.

Cheers,
Aaron