两个日期字段相减，进行计算

lnPact = lnPactBo.getById(lnPact);
  SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
  Date batchDate1=sdf.parse(lnPact.getBatchDate());
  int dueDays1=Integer.parseInt(lnPact.getDueDays());
  Calendar calendar = new GregorianCalendar();
  calendar.setTime(batchDate1);
  calendar.add(Calendar.DATE,1-dueDays1);
  String overdueDate =sdf.format(calendar.getTime());
  lnPact.setOverdueDate(overdueDate);