Good day,
Base to the image I try in input date from June 8 until July 1, but no display colors in my datagridview., I don’t know why.

and here my code

int offset = 4;
                foreach (DataGridViewRow row in dataGridView1.Rows)
                {

                    if (!row.IsNewRow)
                    {

                        if (row.Cells.Count >= 4)
                        {

                            DateTime startBook;
                            DateTime endBook;
                            if (DateTime.TryParse(row.Cells[2].Value?.ToString(), out startBook) &&
                                DateTime.TryParse(row.Cells[3].Value?.ToString(), out endBook))
                            {

                                int startBook_C = startBook.Day;
                                int endBook_C = endBook.Day;
                                int TotalDays = endBook_C - startBook_C;
                                for (int a = 0; a <= TotalDays; a++)
                                {
                                    dataGridView1.Rows[row.Index].Cells[offset + startBook_C - 1 + a].Style.BackColor = Color.Blue;

                                }
                            }
                        }
                    }
                }