I was able to solve this by using the DATEDIFF with a LOOKUP table calculation. With LOOKUP, you can get a value at an offset from the current position (so I used -1 for the previous compared to 0 for the current).
The formula looks like this:
DATEDIFF('day', LOOKUP(MIN([Transaction Date]), -1), LOOKUP(MIN([Transaction Date]), 0))
I used MIN([Transaction Date]) as table calculations require aggregates. I knew in this data that MIN would work (MAX or AVG or ATTR would also work).
I've attached a workbook. Take a look and let me know if you have any questions.
Date Difference.twbx.zip 8.8 KB