Per About's Culinary Arts, the sell-by date on hamburger is intended as a guideline for the stores as to the last day it is reasonable to sell the meat safely. Federal law does not require or enforce a sell-by date.
The sell-by date on hamburger is not a guarantee of the safety of the ground beef. The date is only trustworthy if the meat has been properly handled and kept at optimum temperatures. Regardless of the sell-by date, the U.S. Department of Agriculture urges consumers to cook or freeze hamburger meat within 48 hours of purchasing it to discourage the growth of bacteria in the product.