1、某个时间往推前几天，往后几天

  /**
     * @param time  时间
     *@param day  往前推传入负整数 后退传入正整数
     *@param format  格式
     * @return
     */
public static String afterDay(Date time, Integer day,String format) {
        if (time == null) {
            time = new Date();
        }
        Calendar calendar = Calendar.getInstance();
        calendar.setTime(time);
        //当前往后一天
        calendar.add(Calendar.DATE, day);
        Date date = calendar.getTime();
        DateFormat df = new SimpleDateFormat(format);
        return df.format(date);
    }

2、比较某个时间和当前时间

//比较两个时间
    public static boolean compareDate(String beginDate){
        boolean flag = false;
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        try {
            Date firstDate = sdf.parse(beginDate);
            Date currentDate = new Date();
            String endDate = sdf.format(currentDate);
            Date secondDate = sdf.parse(endDate);
            if(secondDate.getTime()>=firstDate.getTime()){
                flag = true;
            }
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return flag;
    }

3、验证日期格式是否正确

  /**
     * 根据时间 和时间格式 校验是否正确
     *
     * @param length 校验的长度
     * @param sDate  校验的日期
     * @param format 校验的格式
     * @return
     */
    public static boolean isLegalDate(int length, String sDate, String format) {
        int legalLen = length;
        if ((sDate == null) || (sDate.length() != legalLen)) {
            return false;
        }
        DateFormat formatter = new SimpleDateFormat(format);
        try {
            Date date = formatter.parse(sDate);
            return sDate.equals(formatter.format(date));
        } catch (Exception e) {
            return false;
        }
    }

4、获取当前年月日字符串形式

public static String getCurrentStringDate() {
        Calendar instance = Calendar.getInstance();
        int year = instance.get(Calendar.YEAR);
        int month = instance.get(Calendar.MONTH) + 1;
        String _month = String.valueOf(month);
        if (month < 10) {
            _month = "0" + month;
        }
        int currentDay = DateUtils.getCurrentDayOfMonth();
        String _currentDay = String.valueOf(currentDay);
        if (currentDay < 10) {
            _currentDay = "0" + currentDay;
        }
        return year + "年" + _month + "月" + _currentDay+ "日";
    }

5、计算两个日期之间的天数

 /**
     * 在一个时间段计算出多少天
     *
     * @param beginDate 开始时间
     * @param endDate   结束时间
     * @return 天数
     */
    public static Integer daysBetween(String beginDate, String endDate) {
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
            Calendar cal = Calendar.getInstance();
            cal.setTime(sdf.parse(beginDate));
            long time1 = cal.getTimeInMillis();
            cal.setTime(sdf.parse(endDate));
            long time2 = cal.getTimeInMillis();
            long between_days = (time2 - time1) / (1000 * 3600 * 24);
            return Integer.parseInt(String.valueOf(between_days));
        } catch (Exception e) {
            return 0;
        }
    }

6、日期转化为指定字符串格式

public static String date2Str(Date date, String pattern) {
        SimpleDateFormat sdf = new SimpleDateFormat(pattern);
        String format = sdf.format(date);
        return format;
    }

7、计算两个时间相差几个小时，以30分钟为计算基数，即0.5，1，1、、、

public static Double calculationTimeDiff(Date startTime,Date endTime){

        double finalHour = 0;
        long end = endTime.getTime();
        long start = startTime.getTime();
        long diff = end - start;
        long min = diff  /60 / 1000;
        long hour = diff / 60 /60 / 1000;
        if(min%60>=30){
            finalHour = hour+0.5;
        }else{
            finalHour = hour;
        }
        return finalHour;
    }

8、某年某月的天数

public static int getDaysByMonth(String dateStr) {
        if (dateStr.isEmpty()) {
            return 0;
        }
        int day = 0;
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM");
            Date date = sdf.parse(dateStr);
            Calendar instance = Calendar.getInstance();
            instance.setTime(date);
            day = instance.getActualMaximum(Calendar.DAY_OF_MONTH);
        } catch (Exception e) {
            e.printStackTrace();
            return 0;
        }
        return day;
    }