Pop quiz: you grab a jar of mayonnaise from the back of the fridge and notice that its "best by" date was yesterday. Do you throw it out, or use it on your sandwich? Good news: you're safe to use it. The "best by" date has almost nothing to do with how safe it is to consume, and, if stored properly, most condiments last for months after that date. As a general rule, a product's "use by" date is when the manufacturer thinks it might be unsafe to eat, its "best by" date is when they think the product's quality has deteriorated to a point that's no longer appealing, and its "sell by" date is to give stores a guideline for when to rotate their inventory. But because the US has no federal standards for expiration dates, even these definitions are murky. Different states and industry groups have their own regulations for how to label expiration dates, which is why you might see multiple dates on a single product. But even if there were a single standard, expiration dates are rarely a good way to know if food is safe to eat. Expiration dates are all about how long it would take for a foodborne pathogen to multiply to levels that would make you sick, but according to a 2004 paper in the Journal of Food Protection, most pathogens don't really care how long food has been in the fridge. What's more, unexpired dates can also mislead you into thinking food is safe to eat when improper handling and storage could have already made it unsafe. In the end, use your own best judgment: if something smells or tastes off, bulges or molds, or has been out of the fridge for more than two hours, throw it out. Otherwise, follow the guidelines in the accompanying video. We've also collected some other awesome videos on this topic. Watch them now to learn more.