Expiration Date on Milk
There is a lot of variability in how fast milk will go bad.


How long the milk has been opened
Pasteurized vs ultra-pasteurized
Temperature the milk is kept at
Thermal cycling: how long and how often is it kept above 40º
Where in the fridge it is kept. The door will have more thermal cycling than a shelf, higher shelves tend to be warmer than lower ones.


All of these influence how long milk stays good. The big date on the top is used by grocers to determine when it can't be sold. If you look closer there will be text to the effect of, "Use within 7 days after opening". This isn't a guarantee. Leaving the carton out while you cook will allow it to go bad faster, as will keeping it in the door. Also if your fridge is too warm for some reason, things will spoil faster.