1. In a bowl, sift together the flour, baking powder, baking soda and the salt; add 2 tablespoons of the sugar and the butter. Blend the mixture until it resembles meal. Stir in the dates and walnuts. Add the yogurt and combine the mixture well.
2. Mound the dough in the middle of a buttered baking sheet and pat it into an 8-inch round. Sprinkle it with the remaining 1 tablespoon of sugar.
3. Bake the scone in the middle of a 425° F. oven for 15 to 20 minutes, or until it is cooked through. Cut it into 8 wedges. Let the wedges cool on a rack and serve them warm.