How do I see the time left? C#

private void btnStart_Click(object sender, EventArgs e)
       {
           blackRows();
       }

public void blackRows()
       {
           try
           {
               DateTime starttime = DateTime.Now;
               TimeSpan timespent = DateTime.Now - starttime;
               int secondsremaining = (int)(timespent.TotalSeconds / progressBar1.Value * (progressBar1.Maximum - progressBar1.Value));
               int row_count = dataGridView1.Rows.Count;
               progressBar1.Maximum = row_count;
               progressBar1.Step = 1;
               CurrencyManager manager = (CurrencyManager)BindingContext[dataGridView1.DataSource];
               foreach (DataGridViewRow rw in dataGridView1.Rows)
               {
                   if (rw.Cells[2].Style.BackColor == Color.White && rw.Cells[3].Style.BackColor == Color.White && rw.Cells[4].Style.BackColor == Color.White
                       && rw.Cells[5].Style.BackColor == Color.White && rw.Cells[6].Style.BackColor == Color.White && rw.Cells[7].Style.BackColor == Color.White)
                   {
                       manager.SuspendBinding();
                       rw.Visible = false;
                       manager.ResumeBinding();
                       progressBar1.PerformStep();
                       LblContatore.Text = secondsremaining.ToString();
                   }
               }
           }
           catch (Exception)
           {
               MessageBox.Show("Error");
           }
           MessageBox.Show("Completed", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
       }

DateTime starttime = DateTime.Now;
              TimeSpan timespent = DateTime.Now - starttime;
              int secondsremaining = (int)(timespent.TotalSeconds / progressBar1.Value * (progressBar1.Maximum - progressBar1.Value))