Not necessarily. You're right in the case of the vast majority of date tags, where it says "Best Before". That just means what it says, it will be best before that date, but not dangerous after. Other goods will have a "Use by" date, and this should be viewed far more strictly, although a day more wont kill you Stilton has a sell by date because it's illegal to sell a perishable food without one... And the mould that's already in it was grown in a controlled environment - not the same as the mould you'll be growing at the back of your fridge!