Here's the simplest way I can think of:
1. Bring in the data as-is. If you are using a JET data source, Tableau will recognize this as a string.
2. Right-click on the Date field and choose Change Data Type->Date. You now have a usable date.
3. Right-click on the Date field again and choose Create Calculated Field... The Calculated Field dialog opens.
4. Give the new field a name, and use the formula [your date]+27.
5. Click OK. The new date will be created as a dimension in the data that you can use.
Below is a screenshot that shows Jonathan's suggestion.
Once you get Tableau to interpret the [DATE] field as a date, just create a calculated field and add 27 days to get to the date you want:
If you are looking to map this date to the the last day of the month, you can do:
Attached is a workbook that shows this. I added a DATE2 column in excel with the same contents of DATE to make sure the conversion happens as expected.
Date Example.zip 55.2 KB
This is exactly what I was describing, thanks, Pedro!