Need to create a field with Invoice total for last 12 months

Hello everyone,

I need to create a field that has the total invoices related to accounts for the last 12 months is this possible?

The reason I want to create this field is so I can rate the accounts based off of total revenue for that year (bronze, silver, gold). 

I have been trying to come up with a way to do this using calculated fields. Any help would be greatly appreciated.

Thanks in advance! :)