First off "termdate is null and ... termdate< "Start Date"" is mutually exclusive - you won't ever get any data.
Is the "Start Date" the previous month to the current one? Or the previous month to the one in effective date?
If it's previous to the current date, here's what you'll do to calculate it:
NumberVar theMonth;
NumberVar theYear;
if (month(CurrentDate) > 1) then
theMonth := month(CurrentDate) - 1;
else
theMonth := 12;
if (month(CurrentDate) = 1) then
theYear := year(CurrentDate) - 1;
else
theYear := year(CurrentDate)
Date(theYear, theMonth, 15)
-Dell