Having worked in the food industry, I can tell you that sell by dates are hugely conservative - I mean hugely. On some products they can happily be used years after sell by date (the clove oil being a good example, oil really doesn't turn easily - it is used as a preservative for many foods) In most cases it really is a sell by date, to exonerate the supermarket and avoid liability. The only things where I am careful are fresh foods.