these dates are meant to help keep track of the stock in stores. food that has passed its "sell by" or "display until" date, but is still within its use by / best before can still be eaten, if it has been stored correctly. it is common practice in large stores to throw away such food, as it makes the stock control process easier. it also reduces the risk of customers buying food without looking at the date, only to find out the next day that they cannot use it. changing the posted date is illegal in many countries.