We have often wondered why dairy products get two expiration dates here: one for New York City, and one for the rest of the world. Often, the in-city date is a full two or three days earlier than the out-of-city date, which is a pretty significant discrepancy in the life of a quart of half-and-half. Does milk spoil faster in the five boroughs than it does elsewhere?

Yes and no. Thanks to a recent Ask Metafilter post, we found that the ever-reliable Florence Fabricant actually tackled this question in the New York Times way back in 1982. It turns out that NYC has its own rules and dairy dating systems, rules that require shops to stop selling fluid dairy products four days (96 hours) after 6:00 a.m. on the day they were pasteurized, instead of a more common 6-10 day window used in other places. Dairies that sell within the city as well as to other nearby locations double-mark their products–hence the baffling date stamps. The rationale behind the short sale span is simple: sometimes dairy products are left out on the sidewalk in front of stores or are left unrefrigerated in grocery delivery trucks for significant periods of time.

Here’s our take: if you purchase your dairy products from a 24-hour corner store or major chain grocery (places where someone is always around to accept delivery shipments) and bring it home right away, you are likely to have avoided any extended period of non-refrigeration and might want to use the later date as your guide. If, on the other hand, you choose to have your dairy products delivered to your door (as many New Yorkers do), you should probably pay attention to the earlier date. But keep in mind that those dates are the sell-by dates and not the consume-by dates; as Flo Fab writes, you can usually still drink your milk for a few days to a few weeks after the expiration date, as long as it is kept cold and sealed. But if it stinks, pitch it.