Sometimes people want to know whether a work is so old that it it can be copied without any possibility of infringing anyone’s copyright. This post provides the answer (in the United States).
Before January 1, 1979 – when then-existing copyrights were automatically extended to 95 years from the end of the year in which the copyright was secured – copyrights were in effect for 28 years, with extension, if requested, for a second 28-year period (total of 56 years).
So the earliest a work could have been copyrighted (which, at that time, required publication with a copyright notice) and still received the automatic extension was 56 years before January 1, 1979, i.e., January 1, 1923. The copyright for such a work would expire on December 31, 2018 (after 95 years).