I usually use attached script to create date flags, easy to use and to implement.
Create a date_flag table or use parts of the script within your existing table. When you then want to use your flags in the app all you need to do is, Sum({$<_QTD_TY={'1'}>} Sales) and for previous quarter Sum({$<_PR_QTD={'1'}>} Sales) .
Good blog post swuehl but if the data-set is large, would i recommend to do some pre-calculations inside the script e.g. date-flags. Set-analysis in combination with large volumes of data can cause heavy cpu loads on the Qlikview-server.
