public class GreaterThanNullableDate<T> : PropertyValidator
    {
        private Expression<Func<T, DateTime?>> mTarget;
        public GreaterThanNullableDate(Expression<Func<T, DateTime?>> expression)
            : base("Property {PropertyName} greater than another date!")
        {

            mTarget = expression;
        }

        protected override bool IsValid(PropertyValidatorContext context)
        {
            Func<T, DateTime?> oFunc = mTarget.Compile();
            //Type oType = mTarget.Parameters[0].Type;
            DateTime? oTargetDateTime = oFunc.Invoke((T)context.Instance);

            DateTime? oSource = context.PropertyValue as DateTime?;

            if (oSource != null && oTargetDateTime != null)
            {
                if (oSource < oTargetDateTime)
                    return false;
            }

            return true;
        }
    }
    public static class DefaultValidatorExtensions
    {
        public static IRuleBuilderOptions<T, DateTime?> GreaterThanNullableDate<T>(this IRuleBuilder<T, DateTime?> ruleBuilder, Expression<Func<T, DateTime?>> expression) //Expression<PropertySelector<T, DateTime?>> expression)//Expression<Func<T, DateTime?>> expression) 
        {

            return ruleBuilder.SetValidator(new GreaterThanNullableDate<T>(expression));
        }
    }
    
        public partial class NIBRSMasterValidator : AbstractValidator<NIBRSMaster>
    {//only good for NIBRSoffense which means only basic data, nothing about selected code objects. 
        public NIBRSMasterValidator()
        {
            RuleFor(p => p.IncidentDateTime).Must(NotBeFutureDate).WithMessage("NIBRS incident date time: No future date allowed.");
            RuleFor(p => p.IncidentDateTime).GreaterThanNullableDate(p => p.ExceptionalClearanceDateTime).WithMessage("NIBRS incident date cannot be earlier than exceptional clearance date.");
            
        }
        private bool NotBeFutureDate(DateTime? pValue)
        {//where can i move this to? to share. AbstractValidator?

            if (pValue > DateTime.Now)
                return false;

            return true;
        }

    }

Fluent Validation extension method

validation, extension

C#