Preheat oven to 350°F. Grease and flour a 5x9 inch loaf pan. Add flour, dates, granulated sugar, brown sugar, walnuts, butter, baking soda and salt to a mixing bowl. Stir with a fork until dates are well coated and ingredients are blended. Add warm water and stir just until dry ingredients are moistened. Spoon into prepared loaf pan, spread evenly. Bake for 50 minutes or until a cake tester inserted in center comes out clean. Cool in pan on rack for 10 minutes. Remove from pan and cool completely on rack.