Use-by dates are contributing to millions of pounds of wasted food each year, says a new report from the Natural Resources Defense Council and Harvard Law School’s Food Law and Policy Clinic. Apparently, Americans are prematurely throwing out food because of confusion over what expiration dates actually mean.

Food dating began in the 1970s, as Americans began buying more of their food from supermarkets and producing less food at home. “The dates solely indicate freshness, and are used by manufacturers to convey when the product is at its peak,” CNN reports. That means the food does not expire in the sense of becoming inedible on the expiration date marked on its packaging.

Words like “use by” and “sell by” are used inconsistently, contributing to widespread misinterpretation and waste by consumers. “More than 90% of Americans throw out food prematurely, and 40% of the U.S. food supply is tossed—unused—every year because of food dating,” CNN reports.

The FDA lets states come up with their own food dating regulation, so the rules vary widely. Congresswoman Nita Lowey, the senior Democrat on the House Appropriations Committee and author of the Freshness Disclosure Act, says she will be reintroducing legislation to Congress that calls for establishing a consistent food dating system in the United States.

Here’s CNN’s tutorial for understanding common words used on expiration dates:

“Use by” and “Best by”: These dates are intended for consumer use, but are typically the date the manufacturer deems the product reaches peak freshness. It’s not a date to indicate spoilage, nor does it necessarily signal that the food is no longer safe to eat.

“Sell by”: This date is only intended to help manufacturers and retailers, not consumers. It’s a stocking and marketing tool provided by food makers to ensure proper turnover of the products in the store so they still have a long shelf life after consumers buy them. Consumers, however, are misinterpreting it as a date to guide their buying decisions. The report authors say that “sell by” dates should be made invisible to the consumer.

[via CNN]