Is 牛奶 still 安全 to drink if it's two days past the "best by" date?

No
If 你 drink it you'll explode





and die

The short and 安全 answer?
Listen to the cow.


The other answer?
Smell it and look at it, and it also depends whether 或者 not its been opened. I'd say no about drinking the milk- chuck it away, its not 安全 for any person 或者 pet to drink. One 日 你 could get away with, two no.

no.
one time i had some cerial and 浓情巧克力 牛奶 so i poured the 牛奶 in my cup for some 浓情巧克力 牛奶 and just globs came out! (it smelled bad too) it was mayB 3 days past...

That depends on the 牛奶 acutually. 'Sell 由 Date' is the governments doing, which states the law requires the 商店 to sell the products 由 this 日期 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 由 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 你 handle the product (say if 你 keep your eggs in the fridge and need to 移动 them everytime to get something at the back etc).

But with anything 你 buy, fresh/frozen etc must be carefully tested and tried before and after the best 由 date.
I'd say give it a sniff see if it smells like cream 或者 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 由 日期 and sell 由 dates are laws from the government and guidlines only. In England, the government is actually planning on getting rid of the best 由 dates as that is a huge factor in 食物 going to waste that could have been eaten.