Here's a quick and dirty solution:
Create a formula containing something like:
cdatetime(year(dateadd("M",-18,currentdate)),month(dateadd("M",-18,currentdate)),21,0,0,0)
I called it @18 month in the following record selection formula:
(
{Orders.Invoice Date} >= {@18 month}
)
and
(
{Orders.Invoice Date} >= cdatetime(
year(dateadd("M",1,{Orders.Invoice Date})),
month(dateadd("M",1,{Orders.Invoice Date})),
1,0,0,0)-8
and
{Orders.Invoice Date} <= cdatetime(
year(dateadd("M",1,{Orders.Invoice Date})),
month(dateadd("M",1,{Orders.Invoice Date})),
1,23,59,59)-1
)
Note that I based it on being a datetime, you may have to adjust this. if you need further help, flesh out your environment.
-k