通过两个dateTimePicker1日期控件查找数据库日期段的内容，一直没成功？

//获取DateTimePicker日期
            DateTime a = Convert.ToDateTime(dateTimePicker1.Text);
            DateTime b = Convert.ToDateTime(dateTimePicker2.Text);
            string cmdStr = "select * from 历史记录 where 日期/时间 between '" + a + "' and '" + b + "' ";
                while (conn.State != ConnectionState.Open)
                    conn.Open();
                DataSet ds1 = new DataSet();
                OleDbDataAdapter da1 = new OleDbDataAdapter(cmdStr, conn);
                da1.Fill(ds1);
                this.dataGridView1.DataSource = ds1.Tables[0];//数据源里的第一张表

测试了好多种方法都不行？
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