If using whole dates, I would cut them in half, seal up in a sous vide bag with enough water to rehydrate, stew in a 160f bath until they break down and then add toward the end of fermentation. Date paste, on the other hand, would save a few steps. If they were treated with benzoate, it might inhibit further fermentation enough to allow more of the flavor to remain in the finished beer, though I wouldn't attempt that if I were bottling.