RCW 82.87.115: Payment of tax prior to due date.

(1) A taxpayer owing tax under this chapter for a taxable year may submit payment of the tax beginning six months before the due date for filing and payment for that taxable year. Payments must still be in the manner and form prescribed by the department.

(2) Interest may not be assessed for the time period before the due date if the department determines that the taxpayer overpaid the taxes owed for that taxable year and is entitled to a remittance.