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Amanda is a co-founder of Food52.
Well, technically, you should have eaten it by the 12th. If the cheese smells bad or is turning pink and/or moldy, it's no longer good. However, if it looks good and smells good, then it's up to you!
Sorry -- to clarify, I was thinking consume-by not sell-by -- if the sell-by date was 1/12/12, then you have at least a week, and then I'd look for signs of molding/rotting that I describe above.
I would say that if it still looks good or if it was unopened, you could have any where from about 10-15 day. After that I would toss it.