Works fine for me on dummy data.Even if I screw up the dummy data with a period at the end of the text date, it just switches to null. Can you paste what the first 10 rows of your text date field look like?
EDIT: Andy, is the text field at least coming over properly - JET isn't screwing it up somehow is it?
Ok, now this is strange. My laptop had Windows 7 installed and when I re-installed Tableau, the calc now works as expected. This has to be a bug. It's happened to me before too. I think the fix then was to either (1) restart Tableau or (2) reboot the PC.
Thanks for helping out Alex.