In 1907, archaeologist Sir Marc Aurel Stein discovered the Diamond Sutra, a 16-foot scroll containing the Chinese translation of the Sanskrit Buddhist text, from the Caves of the Thousand Buddhas in Dunhuang, China (the discovery in itself is fascinating, but that's a different story).

The Diamond Sutra has the colophon at the inner end that reads:

Reverently made for universal free distribution by Wang Jie on behalf of his two parents on the 15th of the 4th moon of the 9th year of Xiantong.

That date translates to May 11, 868, which makes the Diamond Sutra not only the world's oldest surviving copy of dated printed book, but also the oldest copyright-free /public domain work as well!

Links: High resolution photos of the scroll at the British Library | This Day in Tech