They are best before dates so there is leeway. It is the date where they guarantee the quality, after that you may find a difference with taste and texture, and in our case perhaps the vitamins start to deteriorate. But it isn't that all of a sudden on the specified date that this happens it is over a period of time after the date.
I would happily have packs up to a couple of months after the best by date.... though I might make sure that the first sip is a small one just in case.