Milk often keeps for longer than stated, especially unopened, but I wouldn't want to rely on it even though milk tends to spoil in a way that smells obvious. If My experience of keeping it past its date is accidental, and I don't suggest you repeat it as an experiment. If for some reason I want to maximise storage time of an unopened bottle (perhaps I'm going away for a few days and want it ready for when I get back), I do place it pressed against the back of the fridge, where it's coldest. That guarantees it will still be nice and fresh up to its date, while kept in the fridge door and taken out for use repeatedly the taste might already start to suffer. I wouldn't expect a month though, no more than half that (the bottle in my fridge has 9 days from the day I bought it anyway). So I don't think this is the explanation unless you got unusually lucky.
There are some filtered milk brands designed to keep longer. I think you bought one of these by accident. Your wording suggests you're in the US, but here we have Cravendale. They claim ceramic filters, and 3 weeks shelf life unopened or a week open. That's probably conservative. It's also packed in a white rather than translucent bottle; that may just be marketing. It's a little more expensive than supermarket own brand (80p/litre instead of 68p/litre), but may be worth it if you don't get through much. Walmart offer one called Fairlife (with added vitamins etc.); they don't say how long it keeps unopened, but state 14 days open.
Alternatively you could also buy a smaller bottle, and milk can be frozen.