I found the following information on Impala and Tableau.
NULL value returned
A NULL value is returned when you open a workbook in Tableau 9.0.1 and later and 8.3.5 and later 8.3.x releases that was created in an earlier version and has date/time data stored as a string in a format that Hive doesn't support. To resolve this issue, change the field type back to String and create a calculated field using DATEPARSE() or DATE() to convert the date. Use DATEPARSE() when working with an extract, otherwise use the DATE() function.
Hope this helps!