I'm running PI RC4 on Win7 64-bit platform... both my "Open..." and "Open a copy..." menu functions display a dialog with "Date Modified" for each file -and folder listed. Same story with "Save As...".
When you say you can manually modify this column in the dialog, are you referring to right-clicking on the listing header line to get the drop list of what attributes you can display on that line? I assume that's what you mean - so changing which attributes are checked off for display doesn't persist between restarts of PI?
Just out of curiosity, when you open Windows Explorer, point it at your PI image directory where this problem is occuring, and click View | Choose Details - is "Date Modified" checked off? Is "Date Created" checked off?
Rod