Don't convert it at all in SQL - return the whole DATETIME value, then process it in your presentation language:
decimal d = (decimal) CustomerDetilsdataGridView.Rows[i].Cells[1].Value;
e.Graphics.DrawString(d.ToString("dd/MM/yyyy"), new Font("Arial", 16), Brushes.Black, 120, ysp);
Or
decimal d = (decimal) CustomerDetilsdataGridView.Rows[i].Cells[1].Value;
e.Graphics.DrawString(d.ToString("d"), new Font("Arial", 16), Brushes.Black, 120, ysp);
which will give you the short form date fro that computer's locale.