Make the filling by placing dates, 1/2 cup of the brown sugar, and water in a saucepan. Simmer about 8 minutes, or until mixture thickens, stirring constantly. Remove from heat and cool. Cream margarine the remaining sugar until light. Beat in eggs and vanilla. Sift flour, measure, and sift again with the soda, cream of tartar, and salt. Stir flour mixture into margarine mixture gradually to make a stiff dough. Divide dough in half. Roll out each portion on a lightly floured board, making a rectangle about 8x10 inches and 1/4 inch thick. Spread each with half the cooled date filling. Roll like a jelly roll. Wrap in wax paper and chill thoroughly for 12 hours or overnight. Cut slices 1/4 inch thick with a sharp knife. Place on an ungreased baking sheet. Bake in a 400 oven for 8 to 10 minutes.