I have Excel 2008 installed on my Mac, which I recently upgraded to Mavericks, migrating all my settings across.
Previously (on Mountain Lion) when I entered a date like
11/12 this was interpreted as
11 December, but now Excel insists on interpreting this in American format.
All my existing data (some going back years, originally entered in Windows) displays correctly as I have entered a custom Format
ddd dd/mm/yy, but displays incorrectly in the formula bar.
As far as I can tell, my system settings are correct for Australia - they were migrated across, and I have since checked. Numbers seems to have no problem with dates.