I've dug through everything I can find. I'm feeling really stupid at the moment, but I cannot find a confirmation of that 5pm start time, though it does appear to start today and "immediately." It runs through April 6, but could be extended.

Tweet is from 6:50pm: https://twitter.com/GovInslee/status/1242267557295321090

https://medium.com/wagovernor/inslee-announces-stay-home-sta...

https://www.governor.wa.gov/sites/default/files/proclamation...