Getting an error when trying to return a date field.

Hello,
I have a date field (OrderTimeStamp) in Database which allow nulls. 
When I try to return this field in my LINQ Query I get this following error.
Cannot Convert expression type System.Nullable<System.DateTime> to return type System.DateTime
I do not get this error when I try to return other fileds which also allow nulls. Like FirstName and LastName. Then Why I get error when I try to return a date field?
Following is my LINQ Query.
public static DateTime GetOrderTime(int OrderKey)
        {
            using (var dc = GetDataContext())
            {
                var match = from p in dc.Orders
             & ...