VB 2008 - How to update DATE field on MS_Access Table

I have the following code in order to insert a record on a Access Table:

        con.Open()
        Dim cmd As OleDbCommand = New OleDbCommand("INSERT INTO Cursos (Sigla, Designacao) " _
        & "VALUES (?, ?)", con)
        cmd.Parameters.AddWithValue("@Sigla", txtSigla.Text.ToString)
        cmd.Parameters.AddWithValue("@Designacao", txtDesignacao.Text.ToString)
****        cmd.Parameters.AddWithValue("@DataInicio", txtDataInicio.Text.ToString)                
****        cmd.Parameters.AddWithValue("@DataFim", txtDataFim.Text.ToString)
        cmd.ExecuteNonQuery()
        Me.CursosTableAdapter.Fill(Me.GestaoHorariosDataSet.Cursos)
        con.Close()

But I receive an error. Thefields on the database are defined as date- shortdate.

How an I do it?

Regards