Have you tried the DATE PARSE() function? If you specify the format your date string is in it should convert it to a date field tableau recognizes, which you can then format how you wish.
Your calculation might look like DATEPARSE("YYYYMMDD", <[YourDateString]>)
Hope that helps
Can you share us few examples of how your date looks like. It would be great if you could share your workbook in .twbx format. (If data not to be shared, give us some sample/fake data pls)
Thank you Shravan