Can you try changing the data type from the three dots on the Fields panel on the left hand side of the Dataset page? I have some screenshots below. For the calculation you could try parseDate( DateTime, ‘yyyy-MM-dd’). That way the date formatting is the same as the string in your DateTime field.
I am running into the same issue, but the edit date format option is not working for me.
The date format in my imported column is mm/dd/yyyy and it is currently formatted as string. I validated the column using the format - MM/dd/yyyy - and got the message that the format is valid.
when I try to update after validating, I get this error message.