How to calculate last 18 months in webi 4.0

Hi All,

I need to show last 18 months data in the report.

Ex:

FEB         2014

MAR        2014

APR        2014

MAY        2014

JUN        2014

JUL         2014

AUG        2014

SEP         2014

OCT         2014

NOV         2014

DEC         2014

JAN          2015

FEB          2015

MAR          2015

APR          2015

MAY          2015

JUN          2015

JUL           2015

Regards,

Rooopesh