Current Month:
Sum({<Year=, Quarter=, Month=, Week=, Date={‘>=$(=MonthStart(Max(Date)))<=$(=Max(Date))’}>} Sales)
Previous Month:
Sum({<Year=, Quarter=, Month=, Week=, Date={‘>=$(=MonthStart(Max(Date), -1))<=$(=MonthEnd(Max(Date), -1))’}>} Sales)
Hope this helps you.