Recently we were struggling with import of a spreadsheet into Dynamics 365. If the date field was included, the import would fail. The user doing the import was set to US format, but the date format was set to dd/mm/yyyy format.
We found that we could get past the issue by saving the import spreadsheet as a CSV file. After saving as CSV, the file could be successfully imported, but it could not in XLSX format, even after reformatting the date column.
I’ve noticed that others in the Dynamics Forums have reported issues importing dates via the import utility from xlsx files, but that saving in CSV format works. You may also have success with Wayne Walton’s suggestion to format the dates in the spreadsheet in ISO standard YYYY/MM/DD format.