There are a few ways to go about doing this.
1. If you need all the days of the previous month why not just use a dynamic date filter that uses "between" 1st day and last day.
2. If you want to use database functions you can filter on the Month and use an Applysimple to get the previous month. For example in oracle it would look something like [tt] ApplySimple("to_number(to_char(Add_months((SYSDATE-1),-1), 'YYYYMM' ))",0) [/tt] This qualifies on the Month attribute id and the id has to have the format of YYYYMM
3. Another option is similar to # 2 except you use a between and pick the first date you want and the last day you want using the ApplySimple again. This would qualify on the date attribute, again depending on what form in the attribute you wantto qualify on.