how can i code this function?? C# gridview : so i want to get daily incomings(from gridview) by push a button

    private void button1_Click(object sender, EventArgs e)
        {


            int sum = 0;
            if (DateTime.Today == SalesGridView)
            {
                for (int i = 0; i < SalesGridView.Rows.Count; ++i)
                {
                    sum += Convert.ToInt32(SalesGridView.Rows[i].Cells[3].Value);
                }
                label1.Text = sum.ToString();
            }

.....................................................................................