What's the difference between "best before" and "expired by" for food items?

Definitions and regulation differs per country. Usually, the dating is also closely related to storage instructions which I've mostly left out of my answer to (hopefully) not make things too complex. Here are some of the basics. Skip to my final answer at the bottom if you're not interested in the details. United StatesWhen I researched this topic in more detail, the first thing that struck me was the FDA's stance on the issue of expiration dates: With the exception of infant formula, the laws that the Food and Drug Administration (FDA) administers do not preclude the sale of food that is past the expiration date indicated on the label. FDA does not require food firms to place "expired by", "use by" or "best before" dates on food products. This information is entirely at the discretion of the manufacturer. A principle of U.S. food law is that foods in U.S. commerce must be wholesome and fit for consumption. A "best by", "use by" or expiration date does not relieve a firm from this obligation. A product that is dangerous to consumers would be subject to potential action by FDA to remove it from commerce regardless of any date printed on a label. (http://www.fda.gov/AboutFDA/Transparency/Basics/ucm210073.htm) I'm not exactly sure how that last part is actionable, since all foods go bad at some point. The USDA lists several dates: A "Sell-By" date tells the store how long to display the product for sale. You should buy the product before the date expires. A "Best if Used By (or Before)" date is recommended for best flavor or quality. It is not a purchase or safety date. A "Use-By" date is the last date recommended for the use of the product while at peak quality. The date has been determined by the manufacturer of the product. "Closed or coded dates" are packing numbers for use by the manufacturer. Some dates may be required by the state (like expiration dates on eggs) and there is an entirely different and stricter set of rules for infant formula, but federal law on the issue is sketchy and doesn't say when a food is definitely unfit for consumption. It's usually left to the consumer and some healthy common sense. I haven't found any detailed regulation regarding expiration dates. (http://www.fsis.usda.gov/wps/portal/fsis/topics/food-safety-education/get-answers/food-safety-fact-sheets/food-labeling/food-product-dating/food-product-dating) Time.com concluded that "use by" dates are generally for health and safety reasons while "best before" dates tend to be about quality. (Much like Australia listed below) (http://time.com/103157/food-expiration-dates/) CanadaIn Canada the rules are different. The dating is related to the durable life of a product which is defined as "the anticipated time that an unopened food product, when stored under appropriate conditions, will retain its freshness, taste, nutritional value or any other qualities the manufacturer claims." A "best-before" date tells you when the durable life of a product ends. After that, the product can still be safe to eat, but a "best-before" date does not guarantee product safety. An expiration date is mandatory on formulated liquid diets, foods represented for use in a very low-energy diet, meal replacements (a formulated food that, by itself, can replace one or more daily meals), nutritional supplements and human milk substitutes (infant formula). After the expiry date, the food may not have the same nutrient content declared on the label. Food should not be eaten if the expiration date has passed. They should be discarded. In Canada, a "use by" date has limited applications and behaves similarly to "best before" dates. "Use by" Date The Food and Drug Regulations state the terms "use by" and "employez avant" may replace "best before" for pre-packaged fresh yeast only. It must be presented in the same form and manner as the "best before" date. (http://www.inspection.gc.ca/food/information-for-consumers/fact-sheets/date/eng/1332357469487/1332357545633) AustraliaAustralia appears to have the easiest system with the least amount of exceptions and complications: Foods that must be eaten before a certain time for health or safety reasons should be marked with a use by date. Foods should not be eaten after the use by date and can’t legally be sold after this date because they may pose a health or safety risk. Most foods have a best before date. You can still eat foods for a while after the best before date as they should be safe but they may have lost some quality. Foods that have a best before date can legally be sold after that date provided the food is fit for human consumption. The only food that can have a different date mark on it is bread, which can be labelled with a baked on or baked for date if its shelf life is less than seven days. Foods that have a shelf life of two years or longer, e.g. some canned foods, do not need to be labelled with a best before date. (http://www.foodstandards.gov.au/consumer/labelling/dates/Pages/default.aspx) (http://www.health.qld.gov.au/foodsafety/documents/fs-15-useby.pdf) United Kingdom A product with a ‘use by’ date cannot be sold after that date and should not be used after midnight of that day. The ‘use by’ date applies to products in the state in which they were intended to be sold, for example, a product designed to be sold chilled should not be sold frozen. Where a food requires treatment before consumption, for example, raw meats, the ‘use by’ date means treat e.g. cook, by the end of the date declared. ‘Best before’ dates relate to food quality, including taste, texture, aroma and appearance, whilst ‘use by’ dates relate to food safety. The ‘best before’ date is a quality indication used by the manufacturer to indicate that the food will be, assuming correct storage has been maintained, at its best before a certain date. A food which is past its ‘best before’ date should be safe to eat, but may not be at its best quality after this date. The document also lists a "display until" date like the "sell by" date for retailers. (https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/69316/pb132629-food-date-labelling-110915.pdf) ConclusionThe general consensus appears to be that "Use By" dates are the last date on which the food is safe to consume, assuming it was stored correctly, while "best before" dates are not about food safety, but product quality. To answer the question details: A seller usually wants both dates to be as far in the future as possible. Consumers leave a product on the shelf if they're afraid they can't eat it in time. A longer shelf life means longer potential storage times, less waste of unsold product and more sales. On the other hand, some people will assume a product with a longer shelf life is filled with preservatives and some people will assume those are harmful, which might off-set some of the advantages. Other sources and further reading http://www.fsis.usda.gov/wps/wcm/connect/19013cb7-8a4d-474c-8bd7-bda76b9defb3/Food_Product_Dating.pdf?MOD=AJPERES http://site.foodshare.org/site/DocServer/Food_Storage_and_Shelf_Life_Guidelines.pdf?docID=5822 (mentions Connecticut State Law for date labeling which is required on dairy products only.) http://www.foodsafetynews.com/2013/09/date-labels-on-food-are-unrelated-to-food-safety-and-lead-to-massive-waste/#.VK8DnivF8tM http://healthland.time.com/2013/09/18/is-your-food-expired-dont-be-so-quick-to-toss-it/ http://www.inspection.gc.ca/food/labelling/food-labelling-for-industry/date-markings/eng/1328032988308/1328034259857 http://www.ctvnews.ca/best-before-confusion-leading-to-needless-food-waste-1.744531 http://www.foodauthority.nsw.gov.au/consumers/food-labels/label-facts/best-before-and-use-by-dates#.VK8D2ivF8tM http://www.eufic.org/article/en/artid/Food-labelling-A-wealth-of-information-for-consumers/

What's the difference between "best before" and "expired by" for food items?The FDA website has this to day about the subject of food product dates, "With the exception of infant formula, the laws that the Food and Drug Administration (FDA) administers do not preclude the sale of food that is past the expiration date indicated on the label. FDA does not require food firms to place "expired by", "use by" or "best before" dates on food products. This information is entirely at the discretion of the manufacturer.A principle of U.S. food law is that foods in U.S. commerce must be wholesome and fit for consumption. A "best by", "use by" or expiration date does not relieve a firm from this obligation. A product that is dangerous to consumers would be subject to potential action by FDA to remove it from commerce regardless of any date printed on a label."In many cases, those dates are a marketing gimmick. The company making the food wants to sell you more food, so if you throw it away (because of fears that it's "expired" and therefore somehow unsafe), you will need to buy more. They also want you to eat the food while it has the best freshness and flavor. If you eat it after it's stale and disgusting, you might decide they just make terrible food and decide to purchase from their competitor next time.http://www.fda.gov/AboutFDA/Transparency/Basics/ucm210073.htm

both of them are just estimation of "how long the product will survive" it may survive a bit longer as well but we need to take care of light, temperature, humidity which can improve the life of the product that's why you might have read about warnings like keep away from sunlight or store in a dry place etc. Best before date is usually for food products if not used before that than it may loose its natural taste or color or  can get rancid or spoiled due to moisture.

Both are substitutes. Simply you shouldn't use those items which have passed these dates.