[RESOLVED] [SQL Server] Aggregates in WHERE clause

[nmadd]

SQL Server 9.0.3068

So, I know I cannot put aggregates in a WHERE clause and this is giving me some trouble with what I think should be a simple query. My goal is to display the sum of sales for items by customer for a certain date range. The date range would be the first sale for the item/customer and all sales up to the following 2 months.

use SalesData;
go
select
    s.CustomerName,
    s.ItemNumber,
    sum(s.GrossAmount) GrossSales,
    min(s.DocumentDate) MinDocDate,
    dateadd(month, 2, min(s.DocumentDate)) MaxDocDate
from dbo.SalesByItem s
where
    s.ItemNumber = '12345'
group by
    s.CustomerName,
    s.ItemNumber
order by
    s.CustomerName

I'd love to do something like where s.DocumentDate between MinDocDate and MaxDocDate but I know I can't. I know I can use aggreagates in the HAVING clause but then I'm forced to group by s.DocumentDate which doesn't work as these are individual days and I want to show the sum of sales for the 2 month period.

Can somebody point this SQL newbie in the right direction?