How can I use a variable in webi query for changing the reporting period

Hi,

i'm new in webi and my problem is. My reporting period is for example from 03.01.2014 - 03.31.2014 and for 1 field i want the results from previous month (02.01.2014 - 02.28.2014) and when my reporting period is not a full month for example from 03.01.2014 - 03.15.2014 then is my period from startdate -15 days. The Database is MS SQL 2008. Can i use variables in queries or how can i do this?

Any suggestions? 

Thanks,

Alex