Ok I think I understand a bit better.
What is required here is a derivative of this technique showcased in the ‘Solving Analytical Scenario’ course at Enterprise DNA Online.
It’s called the ‘events in progress’ pattern
You definitely want a date table, but you need to have two ‘inactive relationships’ down to the fact table.
Run through this example, and set up your model this way, then let’s work on the formula if you can’t work it out from here