|Akiba Rubinstein. Credit: www.chessgames.com web site.|
Turns out that the 1/12/1880 date it correct, as proven conclusively by the Ken Whyld Foundation and Association only last month. The entire story, with (of course) the appropriate credit to all involved in the research, is found at the link.
I am not certain where the origin of Rubinstein's alleged "1882" date came from. Michael Negele -- as the wonders if this was due to Rubinstein making himself younger than he was on official documents to escape, or defer, military service. Such "tricks" were quite common among Eastern European Jews at the time, but so far I am not aware of specific proof.