There's a big difference between a 'use by' date and a 'best before' date on perishable goods.
Think of a 'use by' date as an instruction. It's not to be ignored. For food safety reasons you should eat this food by the date shown or throw it away. And it's illegal for retailers to sell foods that are past their 'use by' date for the same reason.
On the other hand, you can think of a 'best before' date as more of a polite suggestion to get the best out of the product. If you eat or drink this product by the date shown it should be in good quality – assuming you've followed the storage instructions. After that date there could be some deterioration, but it won't become unsafe to eat straight away. Some foods deteriorate quite quickly, so you won't want to eat them if they're a week past the date, whereas others, especially canned foods, can last for several years.
Our advice: if in doubt, throw it out!