Hi Veena,
You can do it on report level & universe level.
First on universe create an object called
Date_6_back = add_months([date];-6)
where [Date] is the object on universe having date values.
Now on report create one query , drag the Date_6_back object & provide the prompt for date like (MMYYYY) 062015,072015 etc.
create another query in same report and apply where condition on date object like
date result from another query >= (from query1. take Date_6_back object)
Now on report you will get data from 6 months back to date chosen by users in prompts.
Thanks,
Swapnil