Use-By dates apply to items which, if consumed past the given date, can pose a threat to health.
“Use By dates are an indicator of food safety and the food should not be eaten after this date. Use-by dates are typically present on high risk foods which require refrigeration, such as meat products, fish, pre-prepared foods and dairy products.” 
Most food with a Use-By date needs to be store refrigerated at all times.
When a date is given in calendar format, it is called “open dating”; when it is a code that you need a cipher machine to break, it is called “closed dating” or “coded dating.” “Closed dating” is usually used on shelf-stable items such as tins and boxes.
Use-By dates are based upon assumed handling and storage conditions — the actual span can be shortened by unexpected conditions, such as an electrical outage causing chillers to be without power, etc.
Some consumers wonder if Use-By dates are a ploy by food manufacturers to get us to throw stuff out and buy more, but even bottled water can go stale.
Use-By dates are governed by the General Labelling Requirements of 1996 Food Labelling Regulations in the UK.
Packaged foods must have either a “Use By” or a “Best Before” date on them. The marking or labelling must be indelible, clear and legible. If the information is missing or unintelligible, consumers can complain to the Trading Standards Department.
In the UK, food can be sold after its Best Before date, but retailers will be prosecuted under Section 14 of the Food Safety Act 1990 if the food is unsafe for consumption.
Food, however, cannot be sold past its Use-By date. It is an offence under British law not only to sell it, but even to have it on the shelves in the stores.
In the UK, for eggs, the sell-by date must be at least 7 day before the best-before date, and the best-before date can be no later than 28 days after the laying date. The best-before date on eggs is stamped right on each egg. In the case of eggs, it actually means “use by”, but to conform with European law on eggs, the terminology “best before” has to be used on eggs as an exception instead of “Use-By.”
The last date of the Use-By date means use by midnight of that day, or freeze (if the good is freezable, but if you do, then when you thaw it, use that very day.)
Use-By dates are not required on food which, though perishable, degrades in quality rather than safety — such as most baked goods, butter, margarine, etc. Frozen goods don’t use a Use-By date.
The food items can be labelled “Use by..” followed by an exact date, or “Use by end of…” followed by a month or year.
In France, it is called Date limite de consommation (meaning “end date for consuming.”) It is used for highly perishable food items. It must be accompanied by an understandable calendar date.
A Use-By date doesn’t have to be displayed on fruits, vegetables or live shellfish.
America / Canada
No known national use-by policies in either country.
In North America, “Use By” is treated as a “Best Before” date (indicating quality rather than safety.) Look instead for a date labelled “Expiration Date.”
In America, the expiration date for eggs can be no more than 30 days past when they were packed in cartons.
Consumer advisors say to toss out food if there is an expiration date present that has passed.
Food charities in America such as Second Harvest Food Bank reputedly tell their volunteers sorting non-perishable (tinned, boxed and dried) food to ignore expiration on food items except for baby food, and many consumers do the same for such goods, relying on their eyes and nose to judge food quality and safety after that.
All foods with a shelf life of less than two years must have by law a date stamped on them labelled either “Use-by” or “Best Before.” A Use-By date is used for short-life items, usually fresh, and indicates a date past which the food is not safe to consume. It is illegal to sell items whose Use-By date has passed.
Smithers, Rebecca. Frugal shoppers fuel boom in out-of-date grocery trade. Manchester: The Guardian. 22 January 2010. P. 3.
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