I have this function:

        ' Ajoute un cookie
        ' expC1 Cookie
        ' expC2 Value
        ' expC3 Path
        ' expC4 Expiration
        Public Function AddCookie(ByVal tcCookie As String, ByVal tcValue As String, _
         Optional ByVal tcPath As String = "/", _
         Optional ByVal tdExpiration As Date = Date.Now) As Boolean
            Dim loCookie As New System.Web.HttpCookie(tcCookie)
            loCookie.Value = tcValue
            loCookie.Expires = tdExpiration
            loCookie.Path = tcPath
            oApp.oCookie.Add(loCookie)
        End Function

The optional parameter tdExpiration should have a default value. However, it cannot accept what I did. It has to be a constant. How can I define it?