By Changing the span on each report, I gather that you mean you're either using parameters or hard coding values in the report? And by last 12 months does that mean the last 12 full months, or are you including the current month, and is 12 months from the current date, or back to the 1st of the month?
To use the last 12 months use the Report-Selection Formulas->Record and place something like:
{table.date} >= dateserial(year(currentdate),month(currentdate)-12,1)
and
{table.date} <= dateserial(year(currentdate),month(currentdate),1)-1
This should help you understand the steps, but you may have to make an adjustment based on what your requirements are.
-k