Rolling totals across columns

Is there a way to use a formula (or macro) to calculate a rolling total on columns going across for rows containing category data for a set interval of time?  This would be as opposed to a rolling total on rows going down for columns containing category data.  We track trends for numerous categories (rows) over weeks (columns).  I need to be able to have rolling totals that cover twelve-week intervals whereby as the thirteenth week data is added, the first week drops off, leaving a twelve-week moving total.  I would also like to be able to calculate averages based on the rolling twelfth-week total.

Is this even possible, or do we have to do it all manually?  I appreciate any help!