C Sharp DataGridView Sütunlarındaki Tarih Farkını Hesaplama

using System;
using System.Windows.Forms;

namespace DataGridView
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            dataGridView1.ColumnCount = 4;
            dataGridView1.Columns[0].Name = "Kitap";
            dataGridView1.Columns[1].Name = "Alış Tarihi";
            dataGridView1.Columns[2].Name = "Teslim Tarihi";
            dataGridView1.Columns[3].Name = "Geçen Süre";

            dataGridView1.Rows.Add("Kitap 1", "02.01.2020", "20.01.2020");
            dataGridView1.Rows.Add("Kitap 2", "12.02.2020", "16.03.2020");
            dataGridView1.Rows.Add("Kitap 3", "05.03.2020", "02.04.2020");
            dataGridView1.Rows.Add("Kitap 4", "17.04.2020", "10.07.2020");
            dataGridView1.Rows.Add("Kitap 5", "22.05.2020", "30.09.2020");
            dataGridView1.Rows.Add("Kitap 6", "01.02.2020", "26.10.2020");
            dataGridView1.Rows.Add("Kitap 7", "08.01.2020", "14.03.2020");
            dataGridView1.Rows.Add("Kitap 8", "03.06.2020", "11.12.2020");
            dataGridView1.Rows.Add("Kitap 9", "07.07.2020", "23.11.2020");
            dataGridView1.Rows.Add("Kitap 10", "05.05.2020", "06.06.2020");
        }

        private void button1_Click(object sender, EventArgs e)
        {
            DateTime d1;
            DateTime d2;
            for (int i = 0; i < dataGridView1.RowCount - 1; i++)
            {
                d1 = Convert.ToDateTime(dataGridView1.Rows[i].Cells[2].Value);
                d2 = Convert.ToDateTime(dataGridView1.Rows[i].Cells[1].Value);
                TimeSpan ts = d1 - d2;
                dataGridView1.Rows[i].Cells[3].Value = ts.Days;
            }
        }
    }
}