Well, you would probably have to make two variables called "vQTDStart" and "vQTDEnd" and replace their equivalent values in the statement.
Quarter start and end might be calculated by using the following
LET vQTDStart = quarterstart( Today() ) ; LET vQTDEnd = quarterend( Today() ) ;
You also might want to read up on the MasterCalendar concept. It isn't always useful, but when it is, it saves a lot of time and space and makes Set Analyses more readable.