First, it would probably start from 31 days after December 7thish since your account doesn't start officially until a few days after installation (mine was installed on August 25th and "created" on August 30). Not sure why but maybe one of those lurking CSRs can explain why.
Second, that is an excellent question. I assume SBC DISH would be no different but, with the recent SBC claim that the customers are theirs, I dont know.
If you can't use Club Dish with SBC, that would be a great reason no to go with them. I've got the max 12 credits built up, and it makes my bill next to nothing.