First of all, the milk was produced on different days. But there is a difference between NJ and NY if they have the same expiration date. In NJ it can be sold by the end of that day. In NY the expiration takes place at 12:01am of that day. Don't know why.

Yes, milk produced to be sold within NYC limits carries a local expiration date that's now 3 days earlier than the expiration in surrounding areas. This changed at some point from the old (1980's/1990's) labeling, which used to carry an expiration panel that read something like "May be sold until date stamped on carton. / In NYC may be sold until midnight of previous day." (The original answer refers to that labeling.) My best GUESS (and it is only that) is that NYC has simply passed more strict local laws. Just like you pay more sales tax within NYC limits due to local taxes, grocers are required by the City to remove the milk from the shelves sooner. Why? Because we need it and you don't, honestly. When's the last time you (the original asker) bought a carton of bad milk? I'd wager you could safely buy well PAST the expiration date. Expiration dates TEND to be pessimistic. Which brings me to... In NYC, it's a COMMON occurrence to find sour milk sitting on store shelves. Milk is sold in all manner of tiny, scary shops with odd hours and -- I'm being kind here -- questionable safety practices. (If you've seen how those places are blacked out and shuttered up like underground bunkers at night -- who here /honestly/ thinks they leave the refrigeration running just because they're "required" to by law? And I guarantee those coolers don't stay below 40 degrees in the summer without constant power.) I live in the Bronx, and actually buy most of my milk in Manhattan - even factoring in the subway ride home, it tends to be fresher. At my local "super"market they regularly leave cartons sitting on the shelves for DAYS past _YOUR_ expiration date! Some I bought there were sour immediately after opening -- with a WEEK still left on the carton date! So, basically, the earlier expiration date is just to give us poor (literally) NYC residents a fighting chance at not spending another night shivering and drenched in berry-flavored sweat. (Why DOES sour milk make me sweat berries? I don't even like berries!)