12 month moving sum window

Hi,

I'm trying to conduct the following calculation:

For example - On Q2-2018 there was a total income of 36 , out of which 24 was contributed by clients that started in the last 12 months (Q3-2017).

So generally - I'm trying to know how much income did each client contribute in the last 12 months.

I have date tables and Quarterly start dates on my tables so they can be used.

I made a bunch of attempts that failed, so I guess I'm missing a key piece of knowledge.

I'd really appreciate any help you could offer,

Thank you,

Ohad