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AnotherCat - the CSV catalogues are ad-hoc and ephemeral - IMO a VBA macro would be more easily reused than a cell formula. BTW the results of my inter-book arithmetic ends up in the calibre database via the Import List PI
But the custom column templates I have are trivial, and I don't discern any effect on performance on a 50K book library. So cell formulae and/or VBA macros are not needed, simply format the calibre created date-string column to an Excel date-value and watch them shift to the right
I was wishful thinking that rather than applying the formatting template in custom columns, I could apply it when I create the CSV catalogue, as I could if I were creating an EPUB Catalogue.
Leaving things as is the best solution - as I already said - it works.
BR