What does the expiration date on sour cream refer to--quality or safety?

Sour cream products commonly have a “best by,” or “sell by” date. A “best by” date refers to the last date on which the unopened, refrigerated product is guaranteed to still be at peak quality.  In the case of sour cream, that means it has, up to that point, retained its fresh flavor, texture, aroma, and nutritional values.  This date is about quality, not safety.  Unpleasant as “off” sour cream is to eat, it probably won’t make you sick. The harmful bacteria are destroyed during pasteurization. (But keep sour cream in the fridge at 40° F or slightly cooler so that spoilage bacteria don’t grow and ruin its taste and smell.)
 
A “sell-by” date doesn't tell you, the consumer, how long the product will be good to use. To find out, check out  the guidelines on this website or  the manufacturer's website, or look on the container for a customer service telephone number and call and ask about the product's shelf life. For instance, the website for Daisy Brand sour cream says that the unopened product will stay fresh for up to 2 weeks past the carton’s “sell-by” date. 
 Source(s):
Daisy Brand: Sour Cream FAQs
Cary Frye, vice-president of regulatory affairs, International Dairy Foods Association
Dr. Robert Bradley, professor emeritus of food science, University of Wisconsin