Hi Jonathan,
the way Apptio handles dates is really a two step process.
During the initial upload Apptio will only distinguish between a Label and a Numeric field. What you then need to do in either your raw file or first transform is to set the type of the column to be Date and to provide the format e.g. DD/mm/yy.

If you add the following link after yourdomain.apptio.com/ you will see the help file on the date formats you can use
biit/webhelp/studio/r11.8.3.5.fcf8c159b734.47910/index.html#../Subsystems/Studio/Content/formulas-and-functions/functions/dateformat.htm?Highlight=date format