add column type DateTime

What I want to do is add the column with the same IdExamen that throws the result and subtract me with another of the same type.

this is what I tried without success, someone who has gone through I am, thanks

public JsonResult ContarTiempo(int idExamen)
{
    var SumTiempo = (from p in db.Pregunta
                        where p.IdExamen == idExamen
                        select p.Tiempo).Sum();

    var TotalExamen = (from e in db.Examen
                        where e.IdExamen == idExamen
                        select e.Duracion).FirstOrDefault();

    var TotalPregunta = (dynamic)null;

    try
    {
        TotalPregunta = TotalExamen - SumTiempo;
    }
    catch (Exception)
    {
        TotalPregunta = TotalExamen;
    }
    return new JsonResult { Data = TotalPregunta, JsonRequestBehavior = JsonRequestBehavior.AllowGet };
}