A regulatory authority shall fix rates for water and sewer services for a Class A, B, C, or D utility based on a test year the utility selects that:
(1) includes historic, future, or combined historic and future data;
(2) begins on the first day of a calendar or fiscal year quarter; and
(3) is a consecutive 12-month period that:
(A) begins not later than 18 months after the date the utility files the statement of intent to change rates; and
(B) ends not earlier than 18 months before the date the utility files the statement of intent to change rates.