For the purpose of justifying the reasonableness of a proposed new rate schedule, a utility may utilize either a historical test period of twelve (12) consecutive calendar months or a forward-looking test period of twelve (12) consecutive calendar months consisting of six (6) months of actual historical data derived from the books and records of the utility and six (6) months of projected data which together shall be the period or test year upon which fair and reasonable rates shall be determined by the Arkansas Public Service Commission. However, the commission shall also permit adjustments to any test year so utilized to reflect the effects on an annualized basis of any and all changes in circumstances which may occur within twelve (12) months after the end of the test year where such changes are both reasonably known and measurable.