If you have running totals set up for columns 1 through 12 in the following manner using the formula option and reset on group change and placed in both the detail and group footer
Note: I am assuming a rolling 12 months report with {table.field} as the field being summarized with type of summary of SUM
//#fld1 - Based on currentdate, this is last month
{table.date} in dateserial(year(currentdate),month(currentdate)-1,1)to_ dateserial(year(currentdate),month,1)
//#fld2 - Based on currentdate, this is current month - 2
{table.date} in dateserial(year(currentdate),month(currentdate)-2,1) to_ dateserial(year(currentdate),month(currentdate)-1,1)
...
//#fld12 last year from currentdate
{table.date} in dateserial(year(currentdate),month(currentdate)-12,1) to_ dateserial(year(currentdate),month(currentdate)-11,1)
The following running total has the same type summary will never be reset and placed in the report footer
//#fldtot
{table.date} in dateserial(year(currentdate),month(currentdate)-12,1) to_ dateserial(year(currentdate),month(currentdate),1)
That should do it.
-larry