May be you want:
AC/PlacedDt >= addMonths([%EndOfCurrentMonth%], -13)
I found a workaround that seems to work … How can I display the token on my SELECT clause? I tried to use a microflow to display the value because I wanted to verify what it was, particularly '[%EndOfCurrentMonth%] - 13 * [%MonthLength%]' …. But the microflow didn’t like %MonthLength%?
AC/PlacedDt < '[%BeginOfCurrentMonth%]' and AC/PlacedDt >= '[%EndOfCurrentMonth%] - 13 * [%MonthLength%]'