It’s a movement that thinks that adding the extra 0 gets people to think on a longer time scale. I don’t happen to think it’s a useful trick because we’re not even thinking on thousand-year timescales and a lot of us experienced the turn of the millennia.

It's a meme for Long Now members showing that they intend/expect civilization to continue past the year 9999. It's an effective signal to other members and a great "Ask me" button.

It’s a Long Now Foundation thing: in order for us to have a longer perspective on time, we should prefix years with 0