GA property tax appeals - statutory language limiting comparable sales to year ending on date of valuation? ?

My local board of equalization has indicated that they may only consider sales as being comparable to my property if they were sold in the year preceding the valuation date only and will ignore any sales after the valuation date. I've asked them before for their statutory reference in the OCGA and only received blank stares.

Any idea as to which specific code section limits the definition if comparable sale this way?

Thanks!