Calculating a rolling count of days between two dates if one is empty

It is good practice to use Formulas variables to make Formulas more stupid simple to read.

Stupid simple is good becabuse it may be your own foggy brain needing to adjust the formula a year from now or your successor.

You define the variable and then can refer to it by prefacing the name with a $ sign.

I find it helps to organize my thoughts in a formula flow in a step wise linear fashion, so I typically use them a lot.