copyright-years-regexp

copyright-years-regexp is a variable defined in `copyright.el'.
Its value is

"\\(\\s *\\)\\([1-9]\\([-0-9, ';/*%#\n	]\\|\\s<\\|\\s>\\)*[0-9]+\\)"

Documentation:
Match additional copyright notice years.
The second \( \) construct must match the years.

You can customize this variable.