Preheat oven to 350°F. Line two baking sheets with parchment.
Whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a bowl.
Cream butter with brown and granulated sugars until light and fluffy, 2–3 minutes.
Beat in eggs one at a time, then mix in vanilla until smooth.
Add dry ingredients in two additions, alternating with buttermilk. Mix just until combined.
Fold in chopped dates and nuts. Chill dough 10 minutes to firm slightly.
Scoop heaping tablespoons of dough onto sheets, spacing about 2 inches apart.
Bake 10–12 minutes until set at edges and lightly golden on top.
Cool on the sheet 5 minutes, then transfer cookies to a rack to finish cooling.