C# MS Access数据库日期字段插入空值

private void button1_Click(object sender, EventArgs e)
       {
           OleDbConnection conn = new OleDbConnection(DAL.DBConnectionString.ConnectString);
           OleDbCommand com = new OleDbCommand();
           com.Connection = conn;
           com.CommandText = "insert into aa(a,b) values(@a,@b)";
           com.Parameters.Add(new OleDbParameter("@a", "a1"));
           com.Parameters.Add(new OleDbParameter("@b", dbnull.Value));
           conn.open();
           com.ExecuteNonQuery();        
           conn.Close();

           OleDbCommand com2 = new OleDbCommand();
           com2.Connection = conn;
           conn.open();
           com2.CommandText = "insert into aa(a,b) values(@a,@b)";
           com2.Parameters.Add(new OleDbParameter("@a", "a2"));
           com2.Parameters.Add(new OleDbParameter("@b", Convert.ToDateTime("1992-1-1")));
           com2.ExecuteNonQuery();
           conn.Close();

           OleDbCommand com23 = new OleDbCommand();
           com23.Connection = conn;
           conn.open();
           com23.CommandText = "insert into aa(a,b) values(@a,@b)";
           com23.Parameters.Add(new OleDbParameter("@a", "a3"));
           com23.Parameters.Add(new OleDbParameter("@b", "1993-1-1"));
           com23.ExecuteNonQuery();
           conn.Close();

           MessageBox.Show("OK");
       }