Last 6 Months Data from Value Prompt


I am using Cognos BI 10.2.1.

In my report, I have a Time value prompt having Month, Quarter and Year. There is a crosstab with 5 measures. In my framework, I have a Time dimension with data from 2011 to 2015. However, my Fact table has measure values ONLY from Jan 2011 to Dec 2011. When I select Month from the value prompt, I get all the 12 months data. Everything seems to work fine.

In my new requirement, I need to add another static choice in my prompt, Last 6 Months, in my value prompt. So when I select this, ONLY the data for July 2011 to Dec 2011 should show in the crosstab.

Any help/suggestion will be highly appreciated.

Best Regards