When the marshmallows are properly stored, they can be consumed six months after the best before date. You can eat them safely, after the expiry date has passed, because they have a best before date and not use by date. After the expiry date has passed, you can check to see if they are rotten or spoiled, before consumption. You can check for color change or the texture being different as well. They become sticky and change color, when they have been around for too long.