Two questions, really, the first is: when do people start their sweet potato slips for the coming year? The second is: Are most sweet potatoes in the store of the Georgia Jet variety? (They look like it.)
You can use your zip code to see the planting schedule and then click on sweet potatoes. In the article on them there is a link to determine when to start your slips. I should have started mine last month. Better late than never I guess.
Sweet potatoes are ultra-tropical....even more sensitive to cold than things like tomatoes. They shut down pretty much at any temp. below about 60F. So there is no point in having plants ready to set out too early in the spring, and it needs to be good and warm wherever you intend to sprout the potatoes. Usually I just wait until I see signs of sprouts on them, and I keep some small ones set aside for this purpose since large ones are going to be hard to fit into a pot and they don't like to be cut (whereas white potatoes don't mind so much). Some varieties are temperamental about sprouting at all, and what is more they often seem to vary in this from one year to the next. So usually I keep my sweetpotatoes going by means of living vines kept in a pot as a house plant through winter, as well as by stored potatoes. Come spring I clip up the vines and root them in a tray like any other cuttings. This is often a more reliable way of propagating them, at least for me, than sprouting the tubers.
The slips take a WHILE to develop, like 6-8 weeks I'd say. And as long as you use a whole potato it'll keep producing for many months, so no reason not to get one in water 3-4 months ahead of time as long as you've got a sunny window or a grow light. It's really quite decorative.
The white fleshed ones taste much nicer and are more versatile, unless you like funky carrot flavor that is. You can get cool climate varieties in orange and white, but all the purple ones seem to be tropical, curiously. Wouldn't go with the store ones if you can possibly help it.