Moved from Excel for Mac to Office 365 online; date formats are all international

I was using an Excel spreadsheet on Excel for Mac. I was storing it in the cloud. When my Mac had problems I went to work on it online. I found the document immediately because it was already stored on OneDrive, but all the dates are displayed in international format (DD/MM/YYYY). How can I get these to display in American format? (MM/DD/YY)

Note that I'll be going back to Excel for Mac once I get my Mac straightened out.