As used in this article, the term:

(a) "Billing cycle" shall mean a period of time during the course of which a utility either bills for or measures, by meter- reading or any other means, the usage of its utility services by all of its customers a single time. A utility may elect whether it wishes to determine its billing cycles by date of measurement or by date of billing. A utility which employs twelve billing cycles per year shall be deemed to employ monthly billing cycles. A utility which employs more or fewer than twelve billing cycles per year shall be deemed to employ nonmonthly billing cycles. For a utility employing monthly billing cycles, a billing cycle identified by the name of a particular calendar month must include at least twelve days of that calendar month.

(b) "Billing month" shall have two meanings:

(1) As applied to a utility employing nonmonthly billing cycles and to its customers, a particular "billing month" shall mean the calendar month to which that billing month correlates under section one of this article.

(2) As applied to a utility employing monthly billing cycles and to its customers, a particular "billing month" shall mean the period of customer usage reflected on any bill which, in the case of a utility with billing-date billing cycles, is issued during that particular monthly billing cycle, or for which, in the case of a utility with measurement- date billing cycles, the measurement of usage is made during that particular monthly billing cycle.