The short and безопасно, сейф answer? Listen to the cow.
The other answer? Smell it and look at it, and it also depends whether или not its been opened. I'd say no about drinking the milk- chuck it away, its not безопасно, сейф for any person или pet to drink. One день Ты could get away with, two no.
That depends on the молоко acutually. 'Sell By Date' is the governments doing, which states the law requires the Магазин to sell the products by this дата before they reduce and try and get rid. Two days after reducing and the product is still there and it must be thrown as waste. 'Best By Date' is the manufacturers law, that is put on each product as a GUIDLINE ONLY! Plenty of products, such as cheese, milk, meat, eggs etc can last for a full week AFTER the BBD before going bad. But that depends on storage, how Ты handle the product (say if Ты keep your eggs in the fridge and need to Переместить them everytime to get something at the back etc).
But with anything Ты buy, fresh/frozen etc must be carefully tested and tried before and after the best by date. I'd say give it a sniff see if it smells like cream или stronger then normal and if so take a teaspoon, pour a small amount onto the teaspoon and just touch your tongue to the liquid. If it tastes a little funny then i'd say that's a definate no and to chuck it. But if it tastes okay then i'd say go for it. But like i said, best by дата and sell by dates are laws from the government and guidlines only. In England, the government is actually planning on getting rid of the best by dates as that is a huge factor in Еда going to waste that could have been eaten.