For all of the methods of computation specified in this subsection (2)(a), the billing cycle shall be monthly, and the unpaid principal balance on any day shall be determined by adding to the balance unpaid, as of the beginning of that day, all advances and other permissible amounts charged to the borrower, and deducting all payments and other credits made or received that day. A billing cycle is considered monthly if the closing date of the cycle is on the same date each month, or does not vary by more than four days from that date.
RCW 31.04.115