In this episode, they claim to have crossed time zones once they cross the border between Washington and Idaho. However, the entire panhandle of Idaho, including the whole border with Washington, is in the Pacific time zone.
Comment: Frasier, Niles and Martin are travelling southeast on The Old Oregon Trail (I-84) to Sun Valley, located in south-central Idaho. At one point Niles comments that he's calling from a "Little Red Cabin" in Baker City, Oregon. In other words, they cross into Idaho at the Oregon border, which would put them in the Mountain Time Zone.