Thank you very much, @rfog! There are experts in these forums for nearly everything, sometimes I tend to forget that.
At the moment I have hit a brick wall, because Calibre uses the local time zone for displaying the publishing date in its main window (which is the correct date) but GMT time in export. Since I am not in Amazon’s time zone this means instead of having the publishing dates in the names of the exported files I get the publishing dates minus one day. Which is fatal for daily newspapers, of course.
I’m glad I had discovered this before converting and exporting 3,5k of newspapers.