It seems like this answer should be so easy, but I just can't figure it out.

Here's my situation:

On one worksheet, '2009' I have a series of volumes Jan through December with totals at the end.

On a more recent worksheet, '2010' I have the same exact worksheet, and I'm filling it in as the months go by (currently in July). However, I want to add a column for Year-to-date comparison, and I don't want to have to go to the previous worksheet and add up the months I need.

So, is there a formula that would automatically add the correct months in '2009' based on the data for that month being entered in '2010', e.g. I fill in August in '2010' and on the '2009' worksheet it adds Jan-Aug. etc.

Thanks for your help and time. I just can't figure this one out.