Please can you elaborate a little more on what it is you are trying to achieve, looking at it, your calc and the subsequent function are working correctly.
I suspect the issue you are seeing is a result of the aggregation, (sum),switching the calculation over to an attribute in an expanded view [order id, mdy order date, mdy ship date calc 10 as text] then filtering to individual year of order date you will find the the numbers make sense.
I've put mine through to a window_avg.
Does this help clear it up a bit?
Date Diff.twbx 406.0 KB