Date format convert into numeric value while importing excel using openxml

i am using open xml for impoting excel, all data getting imported easily except date column. the date column converts into numeric value(eg,3278).

        public DataSet ImportExcelData(Stream excelStream, string extension, string keyColumn = null)
        {
            DataSet dsExcelData = null;
            IExcelDataReader excelReader = null;
            try
            {
                if (excelStream == null)
                {
                    return dsExcelData;
                }

                if (extension == ".xls")
                {
                    //1. Reading from a binary Excel file ('97-2003 format; *.xls)
                    excelReader = ExcelReaderFactory.CreateBinaryReader(excelStream);

                }//...
                else if (extension == ".xlsx")
                {
                    //2. Reading from a OpenXml Excel file (2007 format; *.xlsx)
                    excelReader = ExcelReaderFactory.CreateOpenXmlReader(excelStream);
                    
                    var excelDataSet = excelReader.AsDataSet(true);
                  
                    var excelDataSetss = excelReader.GetDateTime(4);

                }//...
                //3. DataSet - The result of each spreadsheet will be created in the result.Tables
                DataSet result = null;
                if (string.IsNullOrEmpty(keyColumn))
                {
                    excelReader.IsFirstRowAsColumnNames = true;
                    result = excelReader.AsDataSet();
                    dsExcelData = result;
                }
                else
                {
                    result = excelReader.AsDataSet();
                    dsExcelData = CopyExcelDataSet(result, keyColumn);
                }
            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                excelReader = null;
            }
            return dsExcelData;
        }