First problem is this line
cell.SetCellValue(row[column].ToString());
You are converting value to string, but you probably convert it to DateTime format, so NPOI knows this is datetime.
or try if this works:
cell.SetCellValue(DateTime.Now);
Second, try first set the style for your cell and then change it DataFormat attribute, but with custom format like this:
IDataFormat dataFormatCustom = workbook.CreateDataFormat();
cell.CellStyle = styles["cell"];
cell.CellStyle.DataFormat = dataFormatCustom.GetFormat("yyyyMMdd HH:mm:ss");
which will format your DateTime value to human readable format.