I think I checked the FDA site, but it was only partially helpful. IIRC, it did distinguish "use by" date from "sell by" date, but it didn't clarify much about what "use by" really means. Does it mean the product is unsafe to use the day after that date or just that its nutritional value and maybe safety decline after that date.   I think they took what I consider an unnecessarily 'conservative' stance, such as insinuating that maybe you should throw out a product on Sept. 15 if the "use by" date was Sept. 14 -- no matter what the product was or how it was stored.
In other words, it wasn't very detailed.