public void RenderPeriod()
        {
            var p = new Period {
                Start = "200401"
            };

            Assert.AreEqual("[200401,∞]", p.ForDisplay());

            p.End = "20050212";
            Assert.AreEqual("[200401,20050212]", p.ForDisplay());

            p.Start = null;
            Assert.AreEqual("[∞,20050212]", p.ForDisplay());
        }