Hi @Kiara_Elliott and welcome to the Forum! 
Sorry to hear you are running into this issue. To be able to import Due Dates with CSV importer, you need to use the standard US date format of month/day/year. You can read more about the CSV Importer in the following Guide article: https://asana.com/guide/help/api/csv-importer
I hope this helps you resolve the issue but if it doesn’t, I would recommend you to reach out to our Support Team with the URL of the Project you are trying to import the data and your CSV file, so they can try to reproduce it and help you resolve this issue!
You can reach out to them following these steps. When you do, feel free to reference this thread so you don’t have to re-explain everything.
Please, let me know if there is anything else I can help you with Kiara! Have a great day!