Excel 2000 conditional formatting query

Hello all, I’m hoping someone out there can help me, as I’m tearing my hair out (and believe me, I don’t have that much to waste!).

I have a spreadsheet set up (Excel 2000), which contains multiple dates in each column, in the format dd/mm/yyyy. Where no date was available, I have entered <undated>.

I’ve used the conditional formatting command: =COUNTIF($B$9:$B$427,B9)>1 to highlight any duplicates within each column.

My problem is, that it will successfully highlight duplicates of <undated>, but it will not highlight duplicate dates.

Can somebody please tell me what I’m doing wrong?

If you need more info on the spreadsheet, just ask!
Thanks,
Michael