Excel dates in Numbers

I have received a .xlsx Excel file. It opens in Numbers, but dates are rendered as integers.

This is the only such failure of translation I have encountered.

Presumably, the problem lies at the originators end (?). If I can't persuade them to format the dates appropriately - they claim they appear in a date format, but I suspect they are just text (?) - how can I transform them to a date format at my end?

Thank you