特定时间后多少天及两日期之间间隔天数

public int getDaysBetween(String strDateStart, String strDateEnd)
   throws ParseException {

  SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
  Date date_start = sdf.parse(strDateStart);
  Date date_end = sdf.parse(strDateEnd);
  Calendar d1 = Calendar.getInstance();
  Calendar d2 = Calendar.getInstance();
  d1.setTime(date_start);
  d2.setTime(date_end);

  if (d1.after(d2)) { // swap dates so that d1 is start and d2 is end
   java.util.Calendar swap = d1;
   d1 = d2;
   d2 = swap;
  }
  int days = d2.get(java.util.Calendar.DAY_OF_YEAR)
    - d1.get(java.util.Calendar.DAY_OF_YEAR);
  int y2 = d2.get(java.util.Calendar.YEAR);
  if (d1.get(java.util.Calendar.YEAR) != y2) {
   d1 = (java.util.Calendar) d1.clone();
   do {
    days += d1.getActualMaximum(java.util.Calendar.DAY_OF_YEAR);
    d1.add(java.util.Calendar.YEAR, 1);
   } while (d1.get(java.util.Calendar.YEAR) != y2);
  }
  return days;
 }

 public String getAfterDay(String sDate, int countdate)
   throws ParseException {

  SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd");
  Date date = df.parse(sDate);

  Calendar c = Calendar.getInstance();

  c.setTime(date);
  c.add(Calendar.DATE, countdate);
  Date d2 = c.getTime();
  String s = df.format(d2);
  return s;
 }