We develop a desktop application that uses some libraries with BSD-license.
BSD licenses got this line: Copyright (c) "year", "copyright holder".
Shall one leave it this way or do I have to fill the "year" and "copyright holder" (in case my program uses a library with a BSD license)?
You have a copyright interest in any work to which you contribute copyrightable content. It is best to indicate this by adding to the lines, so that it goes from
(c) 2015, Fred Yoyodyne
to
(c) 2015, Fred Yoyodyne
(c) 2018, Runtime Error
assuming that is your real name. I have no understanding of the copyright implications of a pseudonymous declaration, but I doubt it will help make things simpler.
Note that, whatever the answer to your question, it remains a question of asserting copyright. That is unrelated to the question of how you license that interest out, so the answer isn't different for BSD, GPL, and so on.
