Webi Prompt with no input

Hi,

  We are using BOXir3 and db is Orable 10g, and I am building a report where we have to show previous and previous-previous months data based on system date when user doesnot input the prompt of Month (format eg: FEB) and Year (format eg: 2016), please help me develop the formulae?

Month as Multiple Selection Variable prompt

Year as Multiple Selection Variable prompt

e.g. when Month and Year prompt is empty, the report is run based on today's date for eg: 21 FEB 2016, the report should show data for JAN 2016 and DEC 2015 else pick the mulilist of values input by user.

Thanks,

Dinya.