Kind of like the Best-By date on many beers. That really tells you nothing. What I want to know is the packaging date.
Most breweries I contacted state beer is fresh for 3 months. You ever go into a store and see beer on the shelf that is at, or past the BB date? I have, and the manager is contacted. The second person contacted is the brewery, to let them know the distributor is not keeping fresh beer on the store shelves.
Regarding yeast, it should remain viable if stored properly, even past the date.
Not much of a factor for us as we harvest our yeast, and brew frequently enough that we always have a fresh slurry ready to go.