I have an old bottle of milk in the refrigerator. In fact it's 12 days past its date. It's perfectly good (or was this morning), because it has been stored in the cold-- it has never been warmed above 36 degrees-- and except for short moments of pouring, it stays sealed so it isn't exposed to dust and bacteria. It was excellent on my cereal this morning. Supermarkets tend to be good at keeping the milk cold, even when it's being loaded, transported, unloaded, and stocked. Smaller stores, not so much. This particular milk came from Whole Foods. Hannaford is good too, and has good extended milk freshness.
So I thought about the woman at work who told me how she always throws out her milk on the "use by" date. I believe this is mistaking the metric, the date, for the product characteristics being measured. The store, or the dairy, compute and specify some average date at which milk will spoil, and prints that on the carton. They want to be safe, and they know milk is not always properly stored. Also, I believe the date on the milk is the "sell by" date, and the product is supposed to stay fresh for a week after that. Regardless, the date is an estimate, and milk can easily go bad sooner if allowed to warm, or be exposed to dust.
And most importantly, please don't mistake a metric for that which is being measured. Sniff your milk. It's fine, it's not impolite, it's almost foolproof, and this exercises some of our oldest natural sensory apparatus. The nose knows.