--------
{Invoices.Type} <> "VOID" and
{Invoices.Date} in LastFullMonth and
{InvoiceItems.Amount} > 0
--------
As you can see.. it is setup to ONLY provide you invoices for the Last Full
Month....
If you have Crystal Reports 6.0 you can modify that formula to do anything
you want... even give you an option to select the beginning date and the
ending date....