how to define column value based on prompted member

[avuong6]

i have a report where i prompt for a period (MAR-16). i would like to define a member to be the DEC month of the prompted value (DEC-16) to use in a another column of my crosstab report. how is this done?
so in my report, column 1 is the user selected period (set with a prompt) and column 2 is always the DEC month of the users selected period.
===============
followup, the report is dimensional. for the 2nd column, i basically want to look at the prompt member in column 1, determine the year of the prompt member and then set the member in column 2 to the DEC member of the prompt year.

[bdbits]

Relational or dimensional? HUGE difference in possible answers...

And are these stored as dates or characters as shown?

[sdf]

if just so happens you have dimensional.
You can add a crasstab space as column and then define contents on the cell.