You can leverage DATEDIFF within an IF statement, like:
DATEDIFF('day',[Target Completion Date],TODAY()) > 5
then "Past Due"
else "Not Past Due"
You can filter out any Nulls--of course, you also have some Null handling calculations (IFNULL, ISNULL, ZN) that can be used to specify how to handle these values.
This is a very basic example, as I'm not very experienced with DATE functions, but you can go as far as you want with it from there.
Thanks but what I'm really looking for is for it to return the number of days past due and return nothing if there was a null value.
I understand. What I provided will help you get there... but returning the number of days will be a little more challenging. Do you intend to include weekend days? Holidays? These things will affect your calculation.
There are many old threads on this forum on how to calculate # of days between dates.
If I am way off base, you may want to upload a packaged workbook with your work thus far, and show us precisely where the problem is.
Something like this should calculate the pure # of days, and return nothing otherwise:
DATEDIFF('day',[Date],TODAY()) > 30 (assumes 30 days previous to today is the cutoff point)
Someone with more experience may be able to provide better assistance; I'm just trying my hand at helping wherever possible on the forum in order to learn more myself (only been using Tableau for a couple of months now myself)
For only weekdays, this thread (or others like it) may provide some guidance: http://community.tableau.com/thread/107647?start=0&tstart=0