NPOI excel读取转datatabel excel日期格式处理

/// <summary>
        /// 将excel中的数据导入到DataTable中
        /// </summary>
        /// <param name="isFirstRowColumn">第一行是否是DataTable的列名</param>
        /// <returns>返回的DataTable</returns>
        public DataTable ExcelToDataTable(bool isFirstRowColumn)
        {
            ISheet sheet = null;
           IWorkbook workbook = null;
           DataTable data = new DataTable();
            int startRow = 0;
            try
            {
                if (fileupload2.FileName.IndexOf(".xlsx") > 0) // 2007版本
                    workbook = new XSSFWorkbook(fileupload2.PostedFile.InputStream);
                else if (fileupload2.FileName.IndexOf(".xls") > 0) // 2003版本
                    workbook = new HSSFWorkbook(fileupload2.PostedFile.InputStream);
                    sheet = workbook.GetSheetAt(0);
                if (sheet != null)
                {
                    IRow firstRow = sheet.GetRow(0);
                    int cellCount = firstRow.LastCellNum; //一行最后一个cell的编号 即总的列数

                    if (isFirstRowColumn)
                    {
                        for (int i = firstRow.FirstCellNum; i < cellCount; ++i)
                        {
                            ICell cell = firstRow.GetCell(i);
                            if (cell != null)
                            {
                                string cellValue = cell.StringCellValue;
                                if (!string.IsNullOrEmpty(cellValue))
                                {
                                    DataColumn column = new DataColumn(cellValue);
                                    data.Columns.Add(column);
                                }
                            }
                        }
                        startRow = sheet.FirstRowNum + 1;
                    }
                    else
                    {
                        startRow = sheet.FirstRowNum;
                    }

                    //最后一列的标号
                    int rowCount = sheet.LastRowNum;
                    for (int i = startRow; i <= rowCount; ++i)
                    {
                        IRow row = sheet.GetRow(i);
                        
                        if (row == null) continue; //没有数据的行默认是null　　　　　　　

                        DataRow dataRow = data.NewRow();
                        for (int j = row.FirstCellNum; j < cellCount; ++j)
                        {
                            if (row.GetCell(j) != null) //同理，没有数据的单元格都默认是null
                                dataRow[j] = row.GetCell(j).ToString();

                            if (row.GetCell(j) != null&&row.GetCell(j).CellType == CellType.Numeric)
                            {
                                //NPOI中数字和日期都是NUMERIC类型的，这里对其进行判断是否是日期类型
                                if (HSSFDateUtil.IsCellDateFormatted(row.GetCell(j)))//日期类型
                                {
                                    dataRow[j] = row.GetCell(j).DateCellValue;
                                }
                            }
                        }
                        data.Rows.Add(dataRow);
                    }
                }

                return data;
            }
            catch (Exception ex)
            {
               LogNet.info("Exception: " + ex.ToString());
                return null;
            }
        }
/// <summary>
        /// 将excel中的数据导入到Data