Finely chop dates and raisins. Put them in a heavy bottomed pot with enough water to cover and leave them to soak overnight. The next day, bring them to a boil in the same water. Simmer over very low heat, stirring occasionally with a wooden spoon to prevent the fruit from burning, and squashing it to a smooth, thick paste against the sides of the pan. Continue cooking and stirring until all the liquid is dissolved, 10 to 15 minutes. Set aside to cool. Remove to a serving bowl. Stir in the wine and garnish with walnuts just before serving.