Combine the flour, baking soda, and salt in a small mixing bowl and set aside.
In a large mixing bowl, using a hand mixer, cream the butter with remaining 1/2 cup granulated sugar and 1/2 cup brown sugar. Mix on medium speed for a minute or two. Add the egg and vanilla and continue mixing until well incorporated.
Add the dry ingredients to the butter mixture and stir with a spoon until combined. Using clean hands, work the dough until it holds together in a ball. Wrap the ball of dough in plastic wrap and refrigerate for an hour.
Meanwhile, place the chopped dates, 1/4 cup granulated sugar and 1/2 cup water in a small saucepan over medium heat. Bring mixture to a boil then reduce heat to low and simmer for about 5 minutes, stirring occasionally. When the dates have cooked down and the mixture is paste like, mix in the chopped nuts and remove the pan from the heat. Set aside to cool.
Roll chilled dough out on a lightly floured piece of wax paper to a 12" x 8" rectangle. Spread the cooled date mixture on the dough to within about 1/2" of the edges. Starting at one of the long ends, begin to carefully roll the dough into a log. Wrap the rolled dough in the wax paper and refrigerate for at least an hour or overnight.
When ready to bake, preheat oven to 375 degrees. Slice roll into 1/4" slices. Place on lightly greased or parchment paper lined baking sheets and bake for 10 to 12 minutes. Remove from oven and allow to cool on baking sheets for a minute or two before transferring to a wire rack to cool completely.