C# 4: Dynamic and Nullable<>

So I've got some code that passes around this anonymous object between methods:

var promo = new
{
    Text = promo.Value,
    StartDate = (startDate == null) ?
        new Nullable<DateTime>() : 
        new Nullable<DateTime>(DateTime.Parse(startDate.Value)),
    EndDate = (endDate == null) ?
        new Nullable<DateTime>() : 
        new Nullable<DateTime>(DateTime.Parse(endDate.Value))
};

Methods that receive this anonymous object type declare its type as dynamic:

private static bool IsPromoActive(dynamic promo)
{
    return /* check StartDate, EndDate */
}

At run-time, however, if StartDate or EndDate are set to new Nullable<DateTime>(DateTime.Parse(...)), a method that receives this dynamic object (named promo) performs this:

if (promo.StartDate.HasValue && promo.StartDate > DateTime.Today ||
    promo.EndDate.HasValue && promo.EndDate < DateTime.Today)
{
    return;
}

It throws an exception:

Server Error in '/' Application.
'System.DateTime' does not contain a definition for 'HasValue' 

What's going on here? What don't I understand about Nullable types and the dynamic keyword?

This code worked fine before I changed I removed the struct that previously stored Text, StartDate, and EndDate and replaced it with an anonymous type and passed it around as dynamic.