Using Nested IF to forecast Revenue, Headcount etc

Hi,

I'm trying to create a nested IF function with conditions. My goal is to have a revenue or expense forecast value in current and future periods. The monthly result would be triggered by start and end dates.
Attached is my sample dataset with a formula starting in cell R4 which is only working for the current month. If I load an end date, I need the end month = 0. In this example, revenue would end in Feb-15, so Mar-15 revenue should be 0.
I'd appreciate any assistance!

Thanks,
Josh