In a large bowl, combine the oats, flour, brown sugar, baking soda and salt. Cut in butter until mixture resembles coarse crumbs. Press firmly into a 13x9 inch, baking pan.
In a large saucepan, combine the dates, sugar and water. (Use an electric hand chopper blender while still cool to puree the dates a little finer if possible. This puree can also be done prior to putting in pan, in a food processor, but this step is not necessary. This makes a smoother topping. Omit this step if a chewier date is preferred.) Cook for 10 minutes or until thickened, stirring frequently. Stir in nuts and extract. Spread over crust.