Adjust oven rack to middle position and heat oven to 350 degrees. Grease 8h by 4h-inch loaf pan. Combine dates, boiling water, and baking soda in medium bowl, cover, and let sit until dates have softened, about 30 minutes.
Whisk whole-wheat flour, all-purpose flour, baking powder, and salt together in large bowl. Whisk buttermilk, sugar, oil, and egg in second bowl until smooth, then stir in dates and their soaking liquid until combined. Using rubber spatula, gently fold buttermilk mixture into flour mixture until just combined. Gently fold in walnuts. Transfer batter to prepared pan and smooth top.
Bake until golden and skewer inserted in center comes out clean, 55 minutes to 1 hour, rotating pan halfway through baking. Let bread cool in pan on wire rack for 10 minutes. Remove bread from pan and let cool completely on wire rack, about 2 hours.
