Would the end result be the median number of days, or a median date (can't rationalize what this would look like)?
If the former then this:
(datediff('day',[Order Create],[Invoice Date]))
and dragged in as a median aggregate should work. You need to have an extract (at least for your current source) to be able to use median.
Median is not one of the choices when I dragged the calculation to the Level of detail shelf.
You need to create a tableau extract to use functions like median
Thanks that worked.