public void AddMonths_creates_new_immutable_DateTime()
{
var jan1st = new DateTime(2014, 01, 01);
var feb1st = jan1st.AddMonths(1);
// first object remains unchanged
Assert.AreEqual(new DateTime(2014, 01, 01), jan1st);
// second object is a new immutable instance
Assert.AreEqual(new DateTime(2014, 02, 01), feb1st);
}