Formula maxes at 36 values?

[leeroyhightower]

I have a spreadsheet that list values for each month of the year for several parts. Some parts have been on the market for 10 years while others are just a few months old whereas the majority fall somewhere in between.

I have 400 parts.

I need to create a formula that sums 36 cells of data from the current month, back 36 months and stops.

Thanks!!