var tss = new[] { new ts { x = 10, dt = new DateTime(2000, 10, 10) },
                  new ts { x = 5, dt = new DateTime(1950, 10, 10) },
                  new ts { x = 30, dt = new DateTime(1900, 10, 10) }
                };
Array.Sort(tss, (x, y) => x.dt.CompareTo(y.dt));