(1) In computing any period of time prescribed by this chapter, the day of the event from which the time begins to run is not included. (For example, if an initial order is served on Friday and the party has twenty-one days to request a review, start counting the days with Saturday.)

(2) As provided in subsection (3) of this section, the last day of the period so computed is included unless it is a Saturday, Sunday, or legal holiday as defined in RCW 1.16.050, in which case the period extends to the end of the next business day.

(3) When the period of time prescribed or allowed is ten days or less, intermediate Saturdays, Sundays and legal holidays shall be excluded in the computation.

(4) The deadline is 5:00 p.m. on the last day of the computed period.