I sometimes freeze milk, so the sell-by date is no indicator of how long it may have been in the fridge.
To point out a key difference between curdled and spoiled, there's not necessarily any smell or texture to indicate that milk has spoiled, and it isn't necessarily 'sour'.
For instance, my senses told me the tall glass I poured from a jug dated six months prior and downed half of in one gulp was regular milk at first, but if you'd blindfolded me, I'd have sworn from the coating left in my mouth and throat after I'd swallowed that I'd just drunk a glass of fish sauce. Its odor wasn't even detectable until the milk had warmed.
I surprisingly didn't get sick at the time, but just thinking about it now gets me very close.