Copyright law already does address the "when does the clock start" issue in great detail (in order to determine when the copyright period expires). We could kee

Copyright law already does address the "when does the clock start" issue in great detail (in order to determine when the copyright period expires). We could keep using all that case law, and just adjust the duration of the copyright down. In particular, making a significant change to a work creates a new copyright for the changed work. If you boot up windows it will say something like "Copyright (C) Microsoft 1983-2013", meaning that the earliest code was written in 1983 and the last significant change was in 2013. That means that this version of Windows will enter the public domain in the year 2014+95. But you can start making copies of the original 1983 version of DOS already in 1983+95.