Recurring Revenue check client in previous period

Hello,

We work with annual recurring revenue (ARR). We want to be able to show our ARR every week and every month. 

To do so I have billing lines per customer per month. 

So for previous months I can sum up all the contractual values (billed values) per month and know our ARR. 

But for the current  month I would need to check wether an existing client was billed or not. 

If he was already billed I can use the ARR from the current month, but if a client isn't billed yet I would need to look back to the previous month and use that amount.

Extra difficulty, and I believe also another calculation, we want to visualize this by week but we bill per month. So I would need to look back at the previous weeks of the current month and if not available look at the previous month....

Could someone help me with this?

Thanks