Hi Vinod
To get the last 13 months in webi report .follow below approach
Assuming you have calendar Month object in the report.
convert your selected date into M/d/yy format . let's say you selected 12/18/13 , your variable should be 12/1/13
variable :
Selected Month =ToDate(FormatDate(useresponse("Date");"Mmm yyyy");"Mmm yyyy")
Flag for month =ToDate([CalMonth];"Mmm yyyy")<=[Selected Month]
Report filter =[Flag for month]+Previous(Self)
select the table you want to filter
add filter -> Report filter between 1 to 13.
Hope this will help.