Does milk remain fresh longer if you never open the container?
This question came up after I noticed that I had a container of milk that was past it’s sell by date, but had never been opened, and therefore had never been exposed to air or bacteria since the moment it was ‘bottled’.
And yes, I realize that the ‘sell by’ date is just that, the date at which a product can’t be sold after, not necessarily the date it will spoil.
So does keeping a milk container unopened prolong it’s expected shelf life?
Why don’t you open it and find out? 
Spoiled milk is not subtle. Open it. If it’s bad, you’ll know.
ETA: The half & half I buy says on the container, “use within 7 days of opening” or some such, but I suspect that has more to do with the high fat content. Still, milk might certainly work the same way.
Longer, yes, but not indefinitely. When it’s opened, it’s exposed to the air and contaminated surfaces (say if you rest the opening on the side of your bowl as you pour), which can lead to faster spoilage. But there are bacteria in the milk already, since pasteurization is not the same as sterilization, so even if it remains unopened and pristine, it will go bad eventually.
It definitely keeps longer unopened. Let me start by explaining that we buy milk in 4 liter lots, consisting of three plastic bags of 1 1/3 liters each in a large plastic bag. The smaller bags are inserted into a plastic container made to fit them, corner is cut off and you pour. When we go away for a week or two and come back, the milk in any already opened container will often taste sour, but the milk in the unopened bags will still be fresh.
Unopened milk definitely stays fresh longer. You can test it by buying two quarts of milk, opening one and using about half of it, then letting it sit in the fridge for 2 weeks. That should give the opened one time to spoil, whereas the sealed one will most likely be fine.
I’ve always wanted a rule-of-thumb on how long milk stays good after you open it, because I buy milk that says “Sell By” a month from now, but when I open it I know I don’t want to be drinking from it a month or even 2 weeks from now.
A good rule of thumb is that it goes bad before I get done using it.
I have had milk go “off” (if not enough that I don’t want to finish it, mainly “soon to be bad milk smell”) after only a couple days on a few occasions, and no, it wasn’t left out or anything (including drinking right from the jug, which can certainly contaminate it); I assume it already had a higher bacterial count before being opened and was well before its sell by date, so it could be considered to be defective. However, I have never had an unopened container go bad, although it is never more than a week before I open it (I go though a gallon every few days). Also, even during a 5 day long power outage one summer (and it was around 100 during the day), an unopened container of milk was actually still good, even though it had warmed up significantly (it didn’t spoil either before being finished off). This is also all simply pasteurized milk.