Each utility in Class C shall keep its books on a monthly basis, so that all transactions applicable thereto, as nearly as may be ascertained, shall be entered monthly in the books of the utility. Each utility in Class D shall keep its books on either a monthly or quarterly basis; when the quarterly basis is used provisions contained herein relative to monthly entries shall be regarded as calling for quarterly entries. A trial balance shall be taken after the monthly or quarterly postings have been made, and each utility shall close its books at the end of each calendar year.
N.Y. Comp. Codes R. & Regs. Tit. 16 § 181.3