http://fairuse.stanford.edu/blog/2010/09/rising-into-the-public-domain.html
For the user in the U.S. or really for anybody in the world, we deem U.S. works pre-1923 as being in the Public Domain. And for the user in the U.S., we also deem non-U.S. works pre-1923 as in the Public Domain. For users outside the U.S., we are fairly conservative with non-U.S. works. I think the date we’re using now is about 1870. It’s a rolling wall, and essentially a best guess. It would be that date for a young author who lived a long time who published something. We use statistical probability, and we roll that wall forward every year.