I think it is because the date is seen as a number or text by Tableau.
1) You can use a calculated formula such as
DATE(LEFT(STR([Date]), 4) + "-" + MID(STR([Date]), 5, 2) + "-" + RIGHT(STR([Date]), 2))
If [Date] is a string, you will not need the STR function:
DATE(LEFT([Date], 4) + "-" + MID([Date], 5, 2) + "-" + RIGHT([Date], 2))
2) or change the field to a date format Tableau recognizes before you import the data.
3) If your source is a text file, you can probably define the date format in a schema.ini file you store in the same folder.
4) I think Tableau will be able to this on the fly in soon to be released Tableau 8.1 (or maybe it is 8.2).
Ps. Print screen of my workbook with your data:
Thank you Kettan!