What does the date on a milk carton refer to?

Cartons of milk sold in the U.S. usually carry either a "sell by" label, indicating how long a retailer can keep a product in the dairy case, or a "use by" date, indicating how long the product will stay fresh at home. This date applies to the refrigerated, unopened product and relates to the milk’s quality, NOT its safety. It tells you how long your milk can be assumed to remain fresh before it develops the food-spoilage bacteria that cause the milk to taste sour, smell bad, and/or lose nutritional value. Pasteurization and other sanitary milk processing methods kill the dangerous pathogenic bacteria that can cause illnesses, said Cary Frye, vice-president of regulatory affairs for the International Dairy Foods Association, which represents the U.S. dairy manufacturing industry.
 
Source(s):
Cary Frye, vice-president of regulatory affairs for the International Dairy Foods Association
International Dairy Foods Association Factsheet: “Pasteurization--Definition and Methods”