From the FDA website (US relevant only though it is likely similar in toher countries):
http://www.fsis.usda.gov/Factsheets/...ting/index.asp
Quote:
Types of 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.
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So the expiration date has nothing to do with the likelyhood that the product will be contaminated or in any way harmful - it merely has to do with the issue of it being at the quality advertised or is set by local laws about how long fresh food (like milk or meat) can be displayed for purchase by a store (who usually takes the food it didn't seel by that date and cooks it to sell in some other form).
I personally don't throw the stuff out unless there are signs of possible contamination. Food doesn't hang around enough for this to be a major concern though.