java求两个日期之间的时间差

public static final String DATE_FORMAT_NORMAL = "yyyy-MM-dd";

//计算给开始日期 结束日期 的间隔天数 yyyy-MM-dd

public static int getDayLength(String start_date,String end_date) throws Exception{

Date fromDate = getStrToDate(start_date,DATE_FORMAT_NORMAL);  //开始日期
Date toDate = getStrToDate(end_date,DATE_FORMAT_NORMAL); //结束日期
long from = fromDate.getTime();

long to = toDate.getTime();

                //一天等于多少毫秒：24*3600*1000

int day = (int)((to-from)/(24*60*60*1000));
return day;

        }

public static Date getStrToDate(String date,String fomtter) throws Exception{
DateFormat df = new SimpleDateFormat(fomtter);
return df.parse(date);
}

public static void main(String[] args) throws Exception{
int days=DateTimeUtil.getDayLength("2016-08-09","2016-08-30");
System.out.println(days);
}

//输出结果21天