"Data type mismatch in criteria expression" SQL UPDATE statement fails

I am using an access database for a very small C# ASP.NET site.  I am trying to perform an update on a record using a parameterized UPDATE statement, but get an error when it is executed. The code is listed below:

        string ID = Request.QueryString["ID"];

        DateTime tempdate = _Date.SelectedDate;
        int hr = Convert.ToInt32(hour.Text);
        if (ampm.SelectedValue == "PM")
            hr += 12;
        int min = Convert.ToInt32(minute.Text);
        DateTime datetime = new DateTime(tempdate.Year, tempdate.Month, tempdate.Day, hr, min, 0);

        if (!Page.IsValid)
            return;

        OleDbCommand cmd = new OleDbCommand("UPDATE tbl_calendar "
            + "SET _Date = @datetime, Title = @title, Type = @typeGroup, Audience = @audience, "
            + "PersonResponsible = @pResponsible, Location = @location, Description = @description "
            + "WHERE ID = @ID", conn);
           
        cmd.Parameters.AddWithValue("@datetime", datetime.ToString());
        cmd.Parameters.AddWithValue("@title", eTitle.Text);
        cmd.Parameters.AddWithValue("@audience", Audience.Text);
        cmd.Parameters.AddWithValue("@typeGroup", _Type.SelectedItem.Value);
        cmd.Parameters.AddWithValue("@pResponsible", PResponsible.Text);
        cmd.Parameters.AddWithValue("@location", location.Text);
        cmd.Parameters.AddWithValue("@description", description.Text);
        cmd.Parameters.AddWithValue("@ID", ID);

        conn.Open();
        cmd.ExecuteNonQuery();
        conn.Close();

Also if I remove the parameter for _Date so it looks like "...SET _Date = '" + datetime + "',..."  The code executes, but then the changes aren't reflected.

On another note, I have an INSERT statement that formats 'datetime' the exact same way, but with a parameter, and it works.  Your help is very much appreciated.

Thank you,

Phil