CREATE TABLE table_79848 ("Rider" TEXT, "Date" TEXT, "Place" TEXT, "Race" TEXT, "Event" TEXT, "Machine" TEXT)

Harry l Stephen rides a Norton machine on what date?
SELECT "Date" FROM table_79848 WHERE "Machine" = 'norton' AND "Rider" = 'harry l stephen'
SELECT "Date" FROM "table_79848" WHERE "Machine" = 'norton' AND "Rider" = 'harry l stephen'