Excel formula for Ageing

Hi,

I am trying to come up with a formula to net down aged data but I am struggling! 

This is a sample from my pivot, I want to run the formula alongside it and what I am trying to do is:

For customer 1 to net off the -735 from 4 Years+ with the 735 in 90-180 so it only shows 735 left in 60-90 days. Then when I refresh the pivot each month these will adjust and then the ageing formula will adjust alongside it.

Thanks,

James