I would like to know if this is possible:

I have two workbooks.. one from this fiscal year and one from last each has 5 reports each of them with 12 months. Currently every month, I pull up last years report and add in the month I am in from last year.. ex: we are in dec of FY 06, so I would go to dec FY 05 and add up sept, oct, nov and dec to get my total. I need to just add a month from the previous year each month. Is there a formula that could do that each month that would just do it when I opened the workbook, if the month had changed??