I just answered a similar question about "out of date" tortellini. Firstly different dates have different meaning, e.g. Sell by date just means that's the last date a shop is allowed to sell it. Best before date is the last day before should still be at peak quality after that it will lose quality, such as not taste quite as nice but not necessarily being harmful. Use by date is a guidance as the last day you should use something, these are often massively exaggerated in order to minimise any chance of legal action against a company in the event of food poisoning etc. also it is likely that it increase sales as the more that is wasted means more is bought.
There was a tv show on a while back (I didn't see it but was told about it by my dad who is pays very little notice to food dates) and in it a food expert attempted to show these dates can often be largely ignored. He and his family were eating food that was several months "out of date".
Milk can go off though, it is unlikely to be particularly harmful but may not taste nice. Some people are very sensitive to off milk, it only has to start to turn for then to notice while others are sensitive at all and it has to really be off to notice.
Your best bet is to go with your own instincts, if it smells or tastes off to you the don't use it otherwise go for it, unless you are cooking for others that will notice it, perhaps get them ti check first, especially if they are sensitive.