If the date on prime rib says sell by 1221 how long is it good until in fridge?

It is not safe to consume prime rib that has a "sell by" date of 1221. The "sell by" date is the last day the product should be sold, and it does not indicate how long the product will be safe to consume after that date. Prime rib is a perishable product that should be refrigerated immediately after purchase and cooked or frozen within a few days. If you have prime rib that has reached or exceeded its "sell by" date, it is best to discard it.