Hi @Editor!
You’ll need to make sure you’re mapping Task Due Date to a date column in the .CSV file. You can see this in the last screenshot of the Asana Guide article - Asana CSV Importer • Asana Product Guide.
Dates need to be formatted in US style as well - MM/DD/YYYY. If you are doing this and it’s not working, I recommend you reach out to our support team. You can do so by visiting Asana Support - Help Center • Asana. Click I’m having trouble with and scroll to the bottom of the page. Then click Let’s talk and you’ll be in touch with our support team. They’ll be happy to help!