You could use a table calculation for this I would start with something like the following:
WINDOW_SUM(COUNT([Account Open Date]))-WINDOW_SUM(COUNT([Account Close Date]))
This calculation sums all of the open dates in the viz, and subtracts the sum of all close dates in the viz from it.
Table calculations are meant to further aggregate already aggregated data, which is exactly what the above calculation is doing. For more information on using and understanding table calculations, I recommend the following resources:
Table Calculations (VIDEO)
Advanced Table Calculations and Formulas (VIDEO)
Making Table Calculations work in Tableau
Want to Learn Table Calculations? Here's How!
Hope this helps!