Although they are called Century Eggs, they take at most a few months to prepare (or you could just buy them at any chinese supermarket). The traditional method to make century eggs was by preserving chicken or duck eggs in a mixture of salt, lime, and ash, then wrapping in the whole package in rice husks and leaving them for several weeks.

After the preservation period, the yolk becomes a dark green / grayish ma.s.s and the egg white becomes a dark brown, transparent solid.

The direct translation of the Chinese term for the Century Egg would be pine flower egg.

Just as a side note, I've eaten them, they taste great, and I'm still alive. No negative side effects have been noted so far. I personally actually really like Century Eggs.