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So this is a bit of a tricky, but as with most things in Tableau it is possible...
I can't do any better job, than the way it is explained in this article
which I have (basically) copied here.
Ensure you follow the instructions exactly!...as we have nested calcs, it is important they are all set up the correct way.
So bring in [Start Date], and set it up (meaning the compute using)
Then [Running Date] and set it up
Then the [isActive]
and finally the [Number of Active Devises]
Let me know if you have any problems