export gridview data to csv

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C#

Hi,
I am creating a windows form using C#. I need to save the data of data gridview on my local drive in csv format.
I have used string builder to save the file. I am pretty much able to do this but there is an issue that when I save my file in csv format it ignores all the formatting done in gridview, e.g. I have set the format of datetime column of gridview as dd/mm/yy hh:mm:ss but after being saved it is showing the date in format dd.mm.yy hh:mm:ss
I don't want to format it manually in excel, I want to make it possible through coding only. I am sending you my export function which I am using.
Any help will be appreciated.

public void Export()
{
  try
  {
    string strColumn = string.Empty;
    string strRow = string.Empty;
    StringBuilder objSB = new StringBuilder();
    for (int i = 0; i < dataGridView1.Columns.Count; i++)
    {
      strColumn += (i >= dataGridView1.Columns.Count - 1) ? dataGridView1.Columns[i].Name : dataGridView1.Columns[i].Name + ",";
    }
    objSB.AppendLine(strColumn);

    for (int i = 1; i < dataGridView1.Rows.Count - 1; i++)
    {
      for (int j = 0; j < dataGridView1.Columns.Count; j++)
      {
        strRow += (j >= dataGridView1.Columns.Count - 1) ? dataGridView1.Rows[i].Cells[j].Value.ToString().Replace("\n", "") : dataGridView1.Rows[i].Cells[j].Value.ToString().Replace("\n", "") + ",";
      }
      objSB.AppendLine(strRow);
      strRow = string.Empty;
    }
    File.AppendAllText(txtFileName.Text, objSB.ToString());
    MessageBox.Show("The CSV File Is Created");
  }
  catch (Exception e)
  {
    MessageBox.Show(e.Message.ToString());
  }
}

Please help me with the proper code as I am quite new in this field.