public void EqualsOk()
        {
            BillingPeriod.CreatePeriod(2015, 4)
            .Should().Be(BillingPeriod.CreatePeriod(2015, 04));

            BillingPeriod.CreatePeriod(2015, 3)
            .Should().NotBe(BillingPeriod.CreatePeriod(2015, 04));
        }
        public void CastToString()
        {
            // Arrange
            BillingPeriod p = BillingPeriod.CreatePeriod(2015, 04);

            // Act
            string s = p;

            // Assert
            s.Should().Be("2015-04");
        }
 public void CreatesCorrectString()
 {
     BillingPeriod.CreatePeriod(3, 9).PeriodId.Should().Be("0003-09");
 }