Preheat the oven to 325 degrees Fahrenheit. To make the cake, mix together the flour, salt, baking soda, cinnamon, allspice, cloves, and tarragon in a large bowl until blended. Set aside. In the bowl of an electric mixer fitted with a paddle attachment, blend together the butter, oil, eggs, and honey at medium speed. Add the yogurt, rum, and vanilla and mix until blended. Gradually add the flour mixture to the bowl, mixing at low speed until just incorporated, taking care not to over mix. Stir in the chopped dates and pour the batter into 3 well-greased and lightly floured 8-inch baking pans. Place in the oven and bake for 30-35 minutes or until set. Remove from the oven and allow to cool for 15 minutes before removing the cake from the pans and placing on a wire rack to cool completely.