之前是自己用 Calendar 类写，比较繁琐，也容易出错。在别人（项目经理）的推荐下，了解了一个专门的工具类

DateDiff

实现方式也是通过 Calendar 的方式

import java.util.Calendar;
import java.util.Date;

public class DateDiff {
    /**
       * 按指定日期单位计算两个日期间的间隔
       *
       * @param timeInterval，
       * @param date1，
       * @param date2
       * @return date1-date2，
       */
    public static synchronized long dateDiff(String timeInterval, Date date1, Date date2) {

        if (timeInterval.equals("year")) {
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date1);
            int time = calendar.get(Calendar.YEAR);
            calendar.setTime(date2);
            return time - calendar.get(Calendar.YEAR);
        }

        if (timeInterval.equals("quarter")) {
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date1);
            int time = calendar.get(Calendar.YEAR) * 4;
            calendar.setTime(date2);
            time -= calendar.get(Calendar.YEAR) * 4;
            calendar.setTime(date1);
            time += calendar.get(Calendar.MONTH) / 4;
            calendar.setTime(date2);
            return time - calendar.get(Calendar.MONTH) / 4;
        }

        if (timeInterval.equals("month")) {
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date1);
            int time = calendar.get(Calendar.YEAR) * 12;
            calendar.setTime(date2);
            time -= calendar.get(Calendar.YEAR) * 12;
            calendar.setTime(date1);
            time += calendar.get(Calendar.MONTH);
            calendar.setTime(date2);
            return time - calendar.get(Calendar.MONTH);
        }

        if (timeInterval.equals("week")) {
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(date1);
            int time = calendar.get(Calendar.YEAR) * 52;
            calendar.setTime(date2);
            time -= calendar.get(Calendar.YEAR) * 52;
            calendar.setTime(date1);
            time += calendar.get(Calendar.WEEK_OF_YEAR);
            calendar.setTime(date2);
            return time - calendar.get(Calendar.WEEK_OF_YEAR);
        }

        if (timeInterval.equals("day")) {
            long time = date1.getTime() / 1000 / 60 / 60 / 24;
            return time - date2.getTime() / 1000 / 60 / 60 / 24;
        }

        if (timeInterval.equals("hour")) {
            long time = date1.getTime() / 1000 / 60 / 60;
            return time - date2.getTime() / 1000 / 60 / 60;
        }

        if (timeInterval.equals("minute")) {
            long time = date1.getTime() / 1000 / 60;
            return time - date2.getTime() / 1000 / 60;
        }

        if (timeInterval.equals("second")) {
            long time = date1.getTime() / 1000;
            return time - date2.getTime() / 1000;
        }

        return date1.getTime() - date2.getTime();
    }
}