RCW 41.50.125

The department may charge interest, as determined by the director, on member or employer contributions owing to any of the retirement systems listed in RCW 41.50.030. The department's authority to charge interest shall extend to all optional and mandatory billings for contributions where member or employer contributions are paid other than immediately after service is rendered. Except as explicitly limited by statute, the director may delay the imposition of interest charges on late contributions under this section if the delay is necessary to implement required changes in the department's accounting and information systems.