Few Steps
1- create a master calendar if you dont have one
https://qlikviewcookbook.com/2015/05/better-calendar-scripts/

2- add a column to your master calendar for quarter year 

e.g. 

'Q' & ceil(month(TempDate) / 3)&'-' & Year(TempDate) AS QuarterYear,

3 - 2 variables to identify current quarter and previous quarter

e.g.

vCurrentQuarter = 'Q' & ceil(month(today()) / 3)&'-' & Year(today())

vPreviousQuarter = 'Q' & ceil(month(addMonths(today(),-3)) / 3)&'-' & Year(addMonths(today(),-3))

4- use the variables in your expressions

e.g.

sum({<QuarterYear={$(vCurrentQuarter)}>}Sales)

sum({<QuarterYear={$(vPreviousQuarter)}>}Sales)