I noticed the resolution status was not being updated by closing issues. Have fixed this by adding an extra post function to the transitions leading up to 'done' in my current workflows. It works well now.
However, before noticing and fixing the problem, we have been closing issues without changing the resolution for over two months. I can change the resolution status in bulk, but I cannot find how to also change the resolution date of the already closed issues. The reason I want to change this is, we use the dashboard gadget "Created vs. resolved chart", which will now display all the closed issues that spread out over JAN/FEB/MAR on this month.
Therefore the question: How can I (preferably in bulk) change the resolution date of closed issues?