I would like to know if there is a way to manage start date and end date for user account or for access to a specific category by a user.
I am developing a plugin for a client that does something similar, except it is linked to paypal, and it updates the role, when expires. That role could be linked to a category, etc.
You could wait for it to be completed, then I discuss your specific requirement, or I can discuss your interest with him and will work somethign out.
grep is your friend.