Make the crust: In a food processor, combine the dates, walnuts, cinnamon and salt. Process into a dough. Spread out onto a tart dish.
Assemble the tart: Spread the yogurt out on top of the date/walnut mixture in an even layer. Arrange the oranges on top, then sprinkle with orange zest. Chill until ready to serve, up to 1 day in advance.