use by dates dont always mean "throw away date"
its how your store your food aswell, if you have a yoghurt in your fridge with a useby date of the 21st july its still possible that it could be fine even after the 24th. it depends how cool your fridge is and in what section of your fridge it was stored in.
i have my fridge set really low because my wife loves ice cold milk so things tend to last a lot longer than they should, i also have a chill department inside my fridge which pretty much freezes anything placed into it (mushrooms in there get covered in ice lol).