Sumproduct between dates

[Ada01]

Hi

I am creating an asset purchase/capitalization plan and in the interest of space and calculation count I have chosen to do a summary monthly depreciation row. I have the first half of the concept down, but how do i get it to Stop after the designated time period (useful life) - Say 3 years (and asset purchased in May 2012 will be done Depr. in May 2015 if it is a three year asset)

=SUMPRODUCT(--($J11:$J23<=O$9)*($M11:$M23))

Thanks