The copyright statement should always correspond to the year that the work was done in.
- work done in 2015, should have ©2015
- work done in 2016 should have ©2016
- files which had work done in 2015 and 2016 should have ©2015–2016
- files which had work done in 2014 and 2016 should have ©2014, 2016
- files which had work done in 2013, 2015 and 2016 should have ©2013, 2015–2016
Your copyright notice should normally look something like:
Copyright © 2016 Anita Developer <firstname.lastname@example.org>
For documentation written in Mallard, you should use the <credit>, <name>, <email> and <years> tags which is generate the correct copyright notice automatically.
The copyright holder will normally be you or, if you make the contribution as part of paid work, then your employer. If you are unsure about this, you should check what your employment contract states on the matter or seek further legal advice.
You must not amend copyright notices which are inserted by other people without their explicit permission which must be recorded appropriately.