For the filling
3 cups coarsely chopped dates
1/4 cup granulated sugar
1 1/2 cups water
For the crust and topping
1 cup firmly packed brown sugar
1 3/4 cups flour
1/2 teaspoon baking soda
1 teaspoon salt
1 1/2 cups old-fashioned rolled oats
3/4 cup (1 1/2 sticks) very cold butter, cut into small cubes
Preheat oven to 400 degrees F. In a nonreactive medium saucepan over low heat, combine dates, granulated sugar, and water. Cook stirring constantly, until thickened, about 15 minutes. Cool completely.
In a large bowl or bowl of an electric mixer, place brown sugar, flour, baking soda, salt, and oats. Mix on low speed until combined. While mixer is running, add butter, 1 piece or 2 at a time. Mix until crumb like, with some lumps of butter no larger than pea size. Using your hands, press and flatten half of crumb mixture into a greased 9 by 13-inch pan. Spread cooled filling over crust. Cover with remaining crumb mixture, patting lightly.
Bake until lightly browned, 25 to 35 minutes. Cool slightly and cut into bars while still in the pan. Gently remove cut bars from pan. Serve warm or at room temperature. Date bars, stored in an airtight container, refrigerate and freeze well.