Accrual Hours

[yann_ep]

Hi all,

I'm trying to find a solution (formula) that will allow me to calculate accrual hours for customers.

Customers can purchase support hours (annually - 15 hours per month). We allow them to accrue unused hours for up to 3 months, then these hours will be forfeited.
Exemple:

Month	Allocation	Used	Delta
Jan	15	11	4
Feb	15 + 4 (from previous month)	17	2
Mar	15 + 2 (from Jan)	12	5
April	15 + 2 (Jan) + 3 (Mar)	20	0
May	15	18	-3 (to be invoiced)
June	15	10	5
July	15 + 5 (June)	12	3
Aug	15 + 5 + 3	11	4
Sept	15 + 5 + 3 + 4	14	1
Oct	15 + 3 + 4 + 1 (hours from June dropped)	...	....

Any help would be great as I've been going around in circles trying to find a workable solution that can be "automated".

Thanks
Yann