Trouble Converting ODS files to XLS

New convert to Mac from Windows. I have a lot of ODS spreadsheets created with OpenOffice. I'd like to move to use Numbers for spreadsheets, but I can't convert the ODS files to XLS without problems.

This screenshot shows a problem, namely that a simple date field doesn't convert properly. What you're seeing here is the spreadsheet opened in OO on the left. On the right, is the file in Numbers. That is, I saved the spreadsheet in XLS format from OO then opened it in Numbers. The dates didn't transfer.

Any idea how to fix that? I'd rather not have to use OO on the Mac.