Who puts the “sell by” and “use by” dates on milk and lactose-free milk?

For the most part, it’s done by the dairy producers themselves, subject to state government regulations. Every carton of milk sold in the U.S. carries 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, according to the National Dairy Council.
 
State expiration label standards, procedures, and regulations vary widely. Cartons of McNeil Nutritionals’ LACTAID® lactose-free milk carry a retail “sell-by” date in compliance with state laws where the milk is produced. The company recommends that consumers use the product by this date.
 
Source(s):
National Dairy Council Factsheet: “Food Safety Information”
Emil Nashed - VP of research and development/quality assurance, Farmland Dairies
Jennifer Minder, Manager, Communications, McNeil Nutritionals, LLC (produces LACTAID® brand lactose-free milk)
McNeil Nutritionals LACTAID® Lactose Free Milk carton