Best Way to Compare Dates

Hi

I have many worksheets with columns of end dates as illustrated on the attached file. I would like to use an Excel function to identify the dates that are not 31-Dec-04 in Column A. I thought I would create a duplicate column (Column B) of 31-Dec-04s and simply use conditional formatting>cell rules>duplicates but this approach seems to identify non 31-Dec-04 values that occur only once. 30-Jun-05 occurs several times in column A and it is not picked up by duplicates.

Thanks in advance for your help.

Al