Не работает выборка по дате

        private void button1_Click(object sender, EventArgs e)
        {
            var yr = DateTime.Today.Year;
            var mth = DateTime.Today.Month;           
            if (radioButton1.Checked)
            {
                string firstDay = new DateTime(yr, mth, 1).ToShortDateString();
                string current_date = DateTime.Now.ToShortDateString();
                string sql = "SELECT type_repair, repair_date FROM fix WHERE repair_date >= '" + firstDay + "' AND repair_date <= '" + current_date + "' ORDER BY repair_date";
                SelectDB2Parse(sql);
            }
            if (radioButton2.Checked)
            {
                string firstDayL = new DateTime(yr, mth, 1).AddMonths(-1).ToShortDateString();
                string lastDayL = new DateTime(yr, mth, 1).AddDays(-1).ToShortDateString();
                string sql = "SELECT type_repair, repair_date FROM fix WHERE repair_date >= '" + firstDayL + "' AND repair_date <= '" + lastDayL + "' ORDER BY repair_date";
                SelectDB2Parse(sql);
            }
            if (radioButton3.Checked)
            {
                string firstDayY = new DateTime(yr, 1, 1).ToShortDateString();
                string current_date = DateTime.Now.ToShortDateString();
                string sql = "SELECT type_repair, repair_date FROM fix WHERE repair_date >= '" + firstDayY + "' AND repair_date <= '" + current_date + "' ORDER BY repair_date";
                SelectDB2Parse(sql);               
            }
        }