How to export to CSV from datagridview with date format

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C#

I need to export from DataGridView to CSV with date format yyyy-mm-dd.

My current export file looks like:
1,24,custodie,8888888,Marius,8940012004412026020,01/01/1900 00:00:00,IN CUSTODIE,01/01/1900 00:00:00,,,,,DISTRIBUITOR,,04/06/2021 00:00:00,,

and i need to look like:

1,24,custodie,8888888,Marius,8940012004412026020,01-01-1900,IN CUSTODIE,01-01-1900 ,,,,,DISTRIBUITOR,,04-06-2021,,

now when i export it takes the date format from windows which is dd / mm / yyyy but i need it to be yyyy-mm-dd without changing the format in windows

this are the cells that need to be formated

writer.Write(rap_cuiclient.Rows[i].Cells[6].Value.ToString() + ",");
writer.Write(rap_cuiclient.Rows[i].Cells[8].Value.ToString() + ",");
writer.Write(rap_cuiclient.Rows[i].Cells[15].Value.ToString() + ",");

What I have tried:

private void ExportGridToCSV1()
        {
            SaveFileDialog sfd = new SaveFileDialog();
            sfd.Filter = "CSV (*.csv)|*.csv";
            sfd.FileName = "Output.csv";
            if (sfd.ShowDialog() == DialogResult.OK)
            {
                TextWriter writer = new StreamWriter(sfd.FileName);


                for (int i = 0; i < rap_cuiclient.Rows.Count - 1; i++)
                {
                    writer.Write(rap_cuiclient.Rows[i].Cells[0].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[1].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[2].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[3].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[4].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[5].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[6].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[7].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[8].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[9].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[10].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[11].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[12].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[13].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[14].Value.ToString() + ",");
                    writer.Write(rap_cuiclient.Rows[i].Cells[15].Value.ToString() + ",");
                    
                    writer.Write(rap_cuiclient.Rows[i].Cells[16].Value.ToString() + ",");

                    writer.WriteLine("");
                }
                writer.Close();
                MessageBox.Show("Date exportate");
            }
        }