Legal Disclaimer. I'm not an attorney. The following should not be construed as legal advice.
They are all following the "rule" that the year in the copyright notice should be the year on which the file was last modified.
A single date on the notice establishes how far back the claim is made (also), so you shouldn't substitute:
Copyright (C) 2015 Bernardo Sulzbach
Copyright (C) 2016 Bernardo Sulzbach
You should let it unchanged (typically your individual copyright will extend from 50 to 100 years beyond your death) or write
Copyright (C) 2015-2016 Bernardo Sulzbach
In the hyphenated form, the first date and the second date serve different purposes:
The last date serves to indicate the latest copyright date for any material in the file
Moreover you should update the copyright year only if you made contributions to the work during that year. So if a file hasn't been updated in a given year, there is no ground to touch the file just to update the year.
For the maintenance burden many editors have a copyright update function. E.g. with emacs you can put:
(add-hook 'before-save-hook 'copyright-update)
~/.emacs init file and have
copyright-update be called automatically every time a file is saved.