How to set up temporary usergroup access?
 

I'm trying to set up my forum where I can add members to a certain usergroup, and they will automatically be removed from that usergroup after a certain number of days (ie: 30 or 60 days). I basically want to run it like a paid subscription would work, but I don't want to charge for it, and I need it to work where members must be added to the usergroup by the admin. How can I set this up?