How many days does milk last after the expiration date posted on the side?

Because I know that milk does expire that exact date. But there's some limit to how much longer it'll last long, and it has to be close to that date.
Update: Because I know that milk doesn't expire that exact date. But there's some limit to how much longer it'll last long, and it has to be close to that date.
Update 2: Okay... I KNOW that milk doesnt expire on the exact date... I am asking how long an OPENED carton of skim milk will last after the date.
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