Merger cell form starttime to stoptime in C#

Here is the code show and load the meeting time data from start to finish time of each room. but don't know how to merge into 1 cell only from the meeting start time to the end meeting time as shown below, hope everyone can support

private void selectAreaColor()
{
    string starttime1 = cboStarttime.Text;
    string stoptime1 = cboStoptime.Text;
    // Get the start time and end time inputs and the selected meeting room
    DateTime startTime = DateTime.ParseExact(starttime1, "HH:mm", null);
    DateTime endTime = DateTime.ParseExact(stoptime1, "HH:mm", null);
    string selectedRoom = cboRoom.SelectedItem.ToString();//  You Can Use this Dynamic Based on Your Selection

    // Loop through the rows in the DataGridView and find the matching row
  
    foreach (DataGridViewRow row in grdMeeting.Rows)
    {
        if (row.Cells[0].Value.ToString() == selectedRoom)
        {
            int rowIndex = row.Index;
            // Loop through the columns and compare the start and end times with the time slots
            for (int i = 1; i < grdMeeting.Columns.Count; i++)
            {
                DateTime columnTime;
                try
                {
                    columnTime = DateTime.ParseExact(grdMeeting.Columns[i].HeaderText, "HH:mm", CultureInfo.InvariantCulture);
                }
                catch (Exception ex)
                {
                    continue;
                }
                //DateTime columnTime = DateTime.ParseExact(grdMeeting.Columns[i].HeaderText, "HH:mm", null);
                string _HeaderText = grdMeeting.Columns[i].Name.ToString();
                if (startTime == columnTime)//|| endTime == columnTime
                {
                   
                    grdMeeting.Rows[rowIndex].Cells[_HeaderText].Style.BackColor = Color.Teal;
                   
                    if (startTime == endTime)
                        return;
                    else
                        startTime = startTime.AddMinutes(30);
                   

                }
            }
        }
    }
}

private void selectAreaColor()
{
    string starttime1 = cboStarttime.Text;
    string stoptime1 = cboStoptime.Text;
    // Get the start time and end time inputs and the selected meeting room
    DateTime startTime = DateTime.ParseExact(starttime1, "HH:mm", null);
    DateTime endTime = DateTime.ParseExact(stoptime1, "HH:mm", null);
    string selectedRoom = cboRoom.SelectedItem.ToString();// You Can Use this Dynamic Based on Your Selection

    // Loop through the rows in the DataGridView and find the matching row
    foreach (DataGridViewRow row in grdMeeting.Rows)
    {
        if (row.Cells[0].Value.ToString() == selectedRoom)
        {
            int rowIndex = row.Index;
            int startColumnIndex = -1;
            int endColumnIndex = -1;
            bool inMeeting = false;

            // Loop through the columns and compare the start and end times with the time slots
            for (int i = 1; i < grdMeeting.Columns.Count; i++)
            {
                DateTime columnTime;
                try
                {
                    columnTime = DateTime.ParseExact(grdMeeting.Columns[i].HeaderText, "HH:mm", CultureInfo.InvariantCulture);
                }
                catch (Exception ex)
                {
                    continue;
                }

                // Check if the current column is within the meeting time
                if (columnTime >= startTime && columnTime < endTime)
                {
                    // If we are not already in a meeting, mark the start column
                    if (!inMeeting)
                    {
                        startColumnIndex = i;
                        inMeeting = true;
                    }
                    endColumnIndex = i;
                }
            }

            // Merge the cells if there is a meeting
            if (inMeeting)
            {
                DataGridViewCell startCell = grdMeeting.Rows[rowIndex].Cells[startColumnIndex];
                DataGridViewCell endCell = grdMeeting.Rows[rowIndex].Cells[endColumnIndex];
                int rowSpan = endColumnIndex - startColumnIndex + 1;
                startCell.RowSpan = rowSpan;
                for (int i = startColumnIndex + 1; i <= endColumnIndex; i++)
                {
                    DataGridViewCell cell = grdMeeting.Rows[rowIndex].Cells[i];
                    cell.Visible = false;
                }
            }
        }
    }
}