(1) A utility filing for an increase in rates and charges shall be prepared to go forward at a hearing on reasonable notice on the data, testimony and exhibits which have been submitted pursuant to these rules and sustain the burden of proof of establishing that its proposed charges are just and reasonable and not unduly discriminatory or preferential or otherwise unlawful. Although ARM 38.5.121 through 38.5.180, inclusive, provide for an historical test period, the utility, in addition, may submit cost of service information for a nonhistorical test period commencing no later than nine months after the close of the historical test period contemplated by those rules.