Can you attach an excel with dummy data or sample workbook with dummy data.
My initial thoughts are that you are bringing the field in as a string (due to the 'N/A' in the data), which you will need to convert to a date. This can be done by changing it to a null;
if [date] = "N/A" then NULL
And then converting this calculated field to a date.
If that doesn't work, as Bharat indicated, appreciate if you could upload a sample .twbx.