I want to have an expression that says...
If [Invoice Date] IS NOT NULL and [Date Paid] IS NULL, then return the name of the month (i.e., March) from [Date of Service] field.
Also, it's possible that invoiced for 5 different months and not one of them has been paid on, therefore the formula above would need to return all 5 month's names...not just one. (i.e., March, April, May, June, July)
Does that make sense?