Birch4, you are correct in your impression. The plateau is generally 7-10 days behind lower adjacent areas in the spring and probably the same number of days ahead of lower areas in the fall. Furthermore, the plateau (which contains Crossville) can be more prone to late spring and early fall frosts than towns such as Dunlap and Dayton in the lower elevations. I'm not familiar with the location of Spencer off-hand. Also expect the plateau to be some 3-6 degrees cooler than the valley at the same latitude, depending on other variables such as exact elevation, aspect, and proximity to water bodies or urban areas (well, urban is probably a misleading term to use for Crossville, even though I would guess that even small cities and towns can alter temperature by a degree or two). Both Dunlap and Dayton typically have long growing seasons relative to many other areas in east Tennessee; I would guess probably 210-220 days between hard frosts on average nowadays. Crossville probably comes in at about 195 (guessing).

I live on the Cumberland Plateau but much lower than Spencer, in Smithville. I've only gardened here a couple years but the 1st and last frost dates for Nashville in the Farmers' Almanac have been pretty dead-on. I pulled out my tomatoes last year when i got overwhelmed and tired of dealing with them, then planted a lovely fall crop of beets, Kale and Collards... All still growing! I didn't pull out my Japanese eggplants till after Halloween and there were perfectly good fruits still attached.