public void IsValid_WhenValueIsNull_ValueSuccessfullValidated()
        {
            //Arrange
            var      validator = new ExpirationDateValidationAttribute(30);
            DateTime?testDate  = null;

            //Act
            var result = validator.IsValid(testDate);

            //Assert
            Assert.IsTrue(result);
        }
        public void IsValid_WhenValueIsNotCorrect_ValueSuccessfullValidated()
        {
            //Arrange
            var validator = new ExpirationDateValidationAttribute(30);
            var testDate  = DateTime.Now.AddDays(29);

            //Act
            var result = validator.IsValid(testDate);

            //Assert
            Assert.IsFalse(result);
        }