The text of the format “Copyright years authors” or similar is a copyright notice which was required by US copyright law until 1989 for copyright protection to apply to a work. Under the Berne Convention, copyright is automatic, and no notice is required. However, this notice still contains the answers to two important questions:
- When was the software published?
- Who are the authors?
The author information is necessary to determine when copyright protection expires. In Germany, this would be seventy years after the death of the last surviving author (see §65 UrhG). Without knowing the identities of the authors, we cannot determine when they died in order to calculate the copyright term. While there are provisions for dealing with anonymous authors (seventy years after publication), this is much more complicated since the identity of the author could be discovered after that time, which would extend the copyright term again. Example: a software is published anonymously in 1990. The copyright term would expire in 2061, and you would then be free to use it in any manner. However, in 2065 the author is identified and they died in 2053. Now, the copyright term extends to 2124 and until then you are no longer free to use the work unless it was licensed to you by the copyright holder.
As I understand it, the years noted only had to be the year of the first publishing of the work in its current form, so it is not necessary to maintain a list of years. After all, a copyright notice isn't a changelog. However, it is still useful to list all years in which a modified version was first published. Contracting consecutive years is commonly understood, e.g. “1999, 2000, 2001” can be written “1999–2001”. However, there should not be any missing years in that range, “1999, 2000, 2001, 2003” can be written as “2003” or “1999–2001, 2003”, but not “1999–2003”. Also, the years record the publishing of a change, not the creation of a change, and not the mere act of publishing. Misunderstanding of these details leads to some quite dishonest copyright notices on some websites that just list the current year, even when the page was not changed. An argument in favour of listing all years rather than just the last year is that different portions of the work were published in different years, which should be reflected in the notice.
In the US, the year of publication determines the copyright duration when the copyright is held by a juridical person such as corporations, so the years are relevant. This applies to GNU software since the respective copyright is assigned to the FSF, but no such practice seems to be possible in Germany.
For GNU software, the appearance of copyright notices is discussed in the chapter Copyright Notices of Information for Maintainers of GNU Software. This permits contracting copyright years as described above if this practice is explained in a README.