After the group rebuilding process, 'Date published' seems to partition splendidly. Thank you

My earliest book is 1853 so I don't have extensive testing of 'by century' and none at all of pre-1000
I'm not sure I would have known to do the group rebuilding if you hadn't warned me. So maybe a note in the Change log (production version) would be a good idea - if you haven't already.