Boy, that's a tough one, as I have no idea how your filter is currently working or what your data looks like.
As a guess, you have a date field that you were filter against, and you were looking for the month to be 1 less than the month is now. I would use the same date field, but I would change the selection criteria to something like:
if {table.datefield} IN cdate(year(currentDate), month(currentDate), 1) to cdate(year(currentDate), month(currentDate), day(currentDate)-1) then
//keep
else
//don't keep
HTH