Many people, and even some people in government, think products automatically go bad once they past the "sell by" date. While it is certainly true for some products, it is not necessarily true for all products. Obviously, perishable products like produce, meat, and dairy should be a concern.
Nevertheless, some products can last forever if they are left unopened. (Twinkies?) How do we know? We have a federal appeals court that tells us so. Basically, the feds prosecuted a guy for allegedly mislabeling (openly – he wasn't sneaking around) salad dressing. Although it was past the sell by date provided by the original retailer, he gave it a new "sell by" date. On appeal after conviction, the US court of appeals states that the original OEM essentially made up the "sell by" date and that the product does not go bad.