Wash. Rev. Code § 84.56.300
On the first Monday of February of each year the county treasurer shall balance up the tax rolls as of December 31 of the prior year in the treasurer's hands and with which the treasurer stands charged on the roll accounts of the county auditor. The treasurer shall then report to the county auditor in full the amount of taxes collected and specify the amount collected on each fund. The treasurer shall also report the amount of taxes that remain uncollected and delinquent upon the tax rolls, which, with collections and credits on account of errors and double assessments, should balance the tax rolls as the treasurer stands charged. The treasurer shall then report the amount of collections on account of interest since the taxes became delinquent, and as added to the original amounts when making such collections, and with which the treasurer is now to be charged by the auditor, such reports to be duly verified by affidavit.
RCW 84.56.300