0CALMONTH Variable -1 OFFSET not working

Hello Experts -

I have a structure and a variable with single-value entry. The variable is on 0CALMONTH.

If user input 12/2014, I wanted to see information of the Previous Month. I'm using a variable offset. But my report output is always for 12/2014. I have data for 05/2007 and 04/2007 also.

In the column header, it shows 11/2014 but there's not data. I am using SAP 7.30 version.

Thanks in advanced!

Cheers,