(a) The accounting treatments specified in this Code section shall apply in any proceeding before the commission to determine the rates to be charged by a gas utility.

(b) In any proceeding to determine the rates to be charged by a gas utility, the gas utility shall file jurisdictionally allocated cost of service data on the basis of a test period, and the commission shall utilize a test period, consisting of actual data for the most recent 12 month period for which data are available, fully adjusted separately to reflect estimated operations during the 12 month period commencing five months from the proposed effective date of the rates. After the initial filing, and until new rates go into effect, the utility shall file actual cost of service data as they become available for each month following the actual data which were filed. The utility shall have the burden of explaining and supporting the reasonableness of all estimates and adjustments contained in its cost of service data.