Place potato in a saucepan, and cover with water; bring to a boil. Reduce heat; simmer 7 minutes. Add plantain; simmer 7 minutes or until very tender, and drain. Return potato and plantain to pan. Add sugar, milk, sour cream, and allspice; beat 2 minutes at high speed of a mixer until smooth. Spoon into a 1 1/2-quart baking dish coated with cooking spray. Sprinkle with dates and almonds. Bake at 350° for 30 minutes or until thoroughly heated.