Combine water, dates, and brown sugar in medium saucepan over medium heat. Bring to a boil, then reduce heat to a simmer, mashing the dates occasionally until they break down and form a fairly thick paste (this takes about 15 to 20 minutes). While dates are cooking down, prepare crust (instructions below).

Let dates cool slightly before pouring over base. 

Prepare 9 x 9 baking pan (grease bottom and sides, line with a band of parchment, if desired).

Combine dry crust ingredients in a large bowl. Either cut or rub margarine into the flour mixture until you have course crumbs. Squeeze a handful to see if it stays together in loose clumps, or falls apart as powder. If too dry, sprinkle with up to 2 tablespoons of water and mix well to incorporate (use just enough to make it form small clumps, not a wet mass). Press half of the mixture into prepared pan.

Pour dates over the prepared base, and spread it until the filling is an even thickness. Top with remaining crust mixture, pressing down lightly to firm.

Bake at 350 degrees for 40 minutes, or until lightly golden. Let cool before cutting.