I've seen something along these lines in the forums before, its the localisation settings in Excel that is the issue, not Tableau.
Peter, we strongly suspect that part of the issue is that it's OpenOffice that's causing part of the problem.
Yep, Its the OpenOffice file that is setup differently I think.
So I have saved the file as cvs from OpenOffice and all seems to work fine now. So I'll just store it as CVS and then I'll be able to work
For some reason automatic date parse kicked in on your workbook and got confused by your dates. Since these are already dates in Excel, automatic date parse should not be used. I can't reproduce this happening again, but may have something to do with changing to string and back, or perhaps changing your Excel file.
Anyway, just remove the table and re-add and it should fix itself.