It seems that DEVONthink doesn’t import the date format from an XLS file saved from LibreOffice.
In the first screenshot, I create a file with a Date column from within LibreOffice.
After adding to DEVONthink, this is how it looks.
The only work-around I see is to save it as a CSV, but that’s a sub-optimal solution and I hope the kind developers at DEVONtechnologies can fix this.
DEVONthink Pro Office 2.6.1
15" MacBook Pro Retina (Mid-2012), 2.6GHz Core i7, 16 GB RAM, 500 GB SSD