In a bowl whisk together sugar, cornstarch, salt, and eggs. In a heavy saucepan heat milk just to boiling and add to egg mixture in a stream, whisking. In pan cook custard over moderate heat, stirring constantly, until it comes to a simmer and simmer, stirring constantly, 2 minutes. Stir in vanilla and dates and chill custard until cold.
Stir in yogurt and freeze ice cream in an ice-cream maker. Stir in walnuts. Ice cream may be made 3 days ahead.
