Offset Value for 0calmonth variable

Dear all,

What i need to show the 2 months billing. First i have a variable called zcalmonth, in first selection i have entered 0calmonth with variable zcalmonth, and in 2nd variable i have 0calmonth with variable and with offset value-1.

However when i run report 0calmonth returns results however 0calmonth-1 returns no value.

I have done that many times, however i guess i am doing something silly.

Any idea? Or anything wrong in variable?

Thanks in advance.