Date glissante calculée

The approach is to create a additional Query using "Add Query" >>  "From Free-hand SQL" then choose a DB connection (relational one, first screenshot). Next write the query there and save the report (you will have the main report query and an additional query at this point). Now again you go to edit the main query and add the date field which you want to filter dynamically. you have to use 'Greater than equal to' and 'Less than equal to' and select the option 'add from another query (Any)'. You will then get list of fields created by other query and can select as per your requirement.

I have shown some screenshots below. Hopefully these will help you to understand what I tried to explain. In case you need any further help I can show you over skype sharing my desktop.

This approach you can use in WebI while creating reports on top of universes having connection type other than OLAP. For example, all the EASIER MOVE universes are of this type (connection type: Relational)... ECC and BW universes as well (have randomly checked some of them).