The 'Date Taken' sort option does nothing when viewing Photo libraries.
Even if all my photos have the same 'taken' date (they don't), I would think that clicking the 'Date Taken' sort option would at least reverse the order. But the galleries remain sorted alphabetically regardless of how many times I click 'Date Taken.'
If I click 'Name' or 'Date Added,' the galleries are sorted accordingly. But if I then click 'Date Taken' again, the galleries return to alphabetic sorting.
My images are from digital cameras, so the Date Taken is automatically embedded in the files.