How do I format my export to excel workbook in microsoft.office.interop.excel?

protected void EXPORT_BUTTON_Click(object sender, EventArgs e)
{
Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
 
// creating new WorkBook within Excel application
Microsoft.Office.Interop.Excel._Workbook workbook  =  app.Workbooks.Add(Type.Missing);
 
String DT1 = "Data table 1";
String DT2 = "Data table 2";
 
ExportToExcel(app, workbook, Gridview1, DT1, 1);
 
ExportToExcel(app, workbook, Gridview2, DT2, 2);   
 
}
public void ExportToExcel(Microsoft.Office.Interop.Excel._Application app, Microsoft.Office.Interop.Excel._Workbook workbook, GridView gridview, string SheetName, int sheetid)
{

 
    // see the excel sheet behind the program
    app.Visible = true;
 
    // get the reference of first sheet. By default its name is Sheet1.
    // store its reference to worksheet

    worksheet = (Excel.Worksheet)workbook,Worksheets.Add();
 
    // changing the name of active sheet
    worksheet.Name = SheetName;
 
    // storing header part in Excel
    for (int i = 1; i < gridview.Columns.Count + 1; i++)
    {
        worksheet.Cells[1, i] = gridview.Columns[i - 1].HeaderText;
    }
 

 
    // storing Each row and column value to excel sheet
    for (int i = 0; i < gridview.Rows.Count - 1; i++)
    {
        for (int j = 0; j < gridview.Columns.Count; j++)
        {
            worksheet.Cells[i + 2, j + 1] = gridview.Rows[i].Cells[j].Text.ToString();
        }
    }
    //save the application
    
    workbook.SaveAs(@"C:\Users\test\Desktop\Test\" + datetime.ToString("dd-MM-yyyy_hh-mm-ss") + ".xls", Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
                }
}