In Windows, click the Start button and start typing set regional format.
You should soon see 'Set regional format' in the search results.
Click on it.
Does the 'Short date' format use - instead of / ?
If so, click 'Change data formats' and select a format using /
If that doesn't help:
You can force Excel to use / by clicking Custom in the Category list, and specify mm/dd/yyyy in the Type box.