how to check if an string contains a particular date in c#

private void buttonAddToRdates_Click(object sender, EventArgs e)
       {
           string connectionString = (@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Documents and Settings\nic-15\My Documents\ALRS.mdb");
           OleDbConnection con = new OleDbConnection(connectionString);
           con.Open();
           OleDbCommand cmd1 = new OleDbCommand("select renewal_dates from arms_records where thana='" + comboBoxthana.Text + "' and licence_number='" + textBoxLicence_number.Text + "'", con);
           OleDbDataAdapter da = new OleDbDataAdapter(cmd1);
           DataSet ds = new DataSet();
           da.Fill(ds);
           int i = ds.Tables[0].Rows.Count;
           if (i == 1)
           {
               DialogResult result = MessageBox.Show("Previous Renewal Dates:"+cmd1.ExecuteScalar().ToString()+"\n Do you want to add: "+dateTimePickerRenewDate.Value.ToShortDateString()+"  to renewal dates?", "Date will be added to Renewal dates.!!!", MessageBoxButtons.YesNo, MessageBoxIcon.Warning);
               if (result == DialogResult.Yes)
                {
               //code for yes
                   ds.Clear();
                   string query = "update arms_records set renewal_dates='" + dateTimePickerRenewDate.Value.ToShortDateString() + "'&',' & renewal_dates where thana='" + comboBoxthana.Text + "' and licence_number='" + textBoxLicence_number.Text + "'";
                   OleDbCommand cmd = new OleDbCommand(query, con);
                   cmd.ExecuteNonQuery();
                   con.Close();
                   MessageBox.Show("Date added to Renewal Dates.\nPlease do enter next renewal data before updating record.");
                 }
           }
               else
               {
                   MessageBox.Show("No record found.");
               }
       }