Do you have a sample workbook for this which you can upload?
Also I see that in the first row Cum_A - Cum_B = -4, so why would the minimum first date be 3/1/2019? Would you mind clarifying a bit further for my understanding.
I assume Cum_A and cum_B are running sums. Create another calc that does Cum_B - Cum_A.
Then do WINDOW_MIN( IF [Cum Diff Calc] <= 1 then ATTR([Date]) END )
Run that last calc the same way you are running the CUM calcs (Probably TABLE(down), but whatever you are doing for those calcs would need to be the same for the MIN calc.)