Milk not spoiled yet [closed]

If it was a closed carton of UHT milk, with a "Best before" date mentioned on the carton, it's not at all odd that the milk isn't spoiled three weeks after the date on the package.

Once the carton is opened, the milk is usually contaminated quickly, and will spoil in a few days. But how long that takes depends on a lot of factors (kept cold, handled in a clean environment, not left standing open, all will help preventing spoilage).

There are two different mentions possible on pre-packaged food products.
One is "Use by" followed by a date, this is used for e.g. pasteurised milk, and indicated that the product might be spoiled after that data. In practise, there's some margin depending on how the product was treated; if it is kept at the proper temperature all the time, the product will keep longer.
The other possible mention is "Best before" + a date. This is used for dry, sterilised or otherwise conservated products that won't undergo microbial spoilage as long as the packeging is unopened. Such products are perfectly safe to eat even after the date mentioned, but flavour, texture or colour might not be optimal anymore (e.g. the oils in nuts can become rancid after prolonged storage, while still being safe to eat/use).