Step 1Heat the oven to 375 degrees. Grease a 13- by 9-inch baking pan and line with parchment. Grease the parchment and set aside.
Step 2In a large bowl, whisk together the flours, baking powder and soda, and salt. In a separate bowl, rub the orange zest into the granulated sugar.
Step 3In the bowl of a stand mixer using the paddle attachment, cream both sugars and the butter until light and airy, 3 to 5 minutes. Beat in the eggs, one at a time, until fully incorporated, scraping down the sides after each addition. Beat in the vanilla.
Step 4Add the dry ingredients in 2 additions, scraping down the sides, and mixing until completely combined. Stir in the dates and nuts.
Step 5Spoon the batter into the pan, spreading it evenly. Bake until set in the middle, about 1 hour, rotating halfway through for even baking. Cool before slicing.