Spire.XLS 14.3.3 ではピボットテーブルのグループ化機能がサポートされています

XlsPivotTable pt = worksheet.PivotTables[0] as XlsPivotTable;
IPivotField field = pt.RowFields[0];
DateTime start = new DateTime(2024, 5, 6);
DateTime end = new DateTime(2024, 10, 6);
PivotGroupByTypes[] types = new PivotGroupByTypes[]{ PivotGroupByTypes.Days };
field.CreateGroup(start, end, types, 1);

Workbook workbook = new Workbook();
workbook.LoadFromFile(@"test.xlsx");
Worksheet sheet = workbook.Worksheets[0];
sheet.Range["A1"].FindAll()

foreach (XlsPivotTable pt in wb.Worksheets["PivotTable"].PivotTables)
 {
 pt.Options.RepeatAllItemLabels = true; 
 }