Hello Will,
Here's an approach
Create two Dimensions
[FirstDayOfLastMonth] =RelativeDate(RelativeDate(CurrentDate();-DayNumberOfMonth(CurrentDate()));-DayNumberOfMonth(RelativeDate(CurrentDate();-DayNumberOfMonth(CurrentDate())-1)))
[LastDayOfLastmonth] =RelativeDate(CurrentDate();-DayNumberOfMonth(CurrentDate()))
Then finally [VarYesNo]=If [DateObject] Between([FirstDayOfLastMonth];[LastDayOfLastmonth]) Then "Yes" Else "No"
Regards
Niraj