Java date calculation (the remaining days of the month, the number of days in the month to get the date)

In daily development, you will encounter calculations about dates, such as: the number of days in the current month, the number of days between two dates, the remaining days in the current month, etc...

Below is a demo about date calculation, which will be continuously updated later...

1. Get the number of days in the current month or the total number of days in a month

    /**
     * 获取日期当月的天数
     * @param dateStr yyyy-MM 或者yyyy-MM-dd
     * */
    public static int getDaysByDate(String dateStr){
        int year= Integer.parseInt(dateStr.substring(0,4));
        int month=Integer.parseInt(dateStr.substring(5,7));
        Calendar c = Calendar.getInstance();
        c.set(year, month, 0);
        return c.get(Calendar.DAY_OF_MONTH);
    }


    public static void main(String[] args) {
        int daysByDate = getDaysByDate("2023-01");
        System.out.println("2023年1月总天数:"+daysByDate);
    }

The test example returns results:

2. The remaining days of the month

    /**
     * 当月剩余天数
     * @param date 格式yyyy-MM-dd
     * */
    public static Integer monthEndNum(String date){
        SimpleDateFormat format=new SimpleDateFormat("yyyy-MM-dd");
        Date dateTime = null;
        try {
            dateTime = format.parse(date);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        Calendar c = Calendar.getInstance();
        c.setTime(dateTime);
        int today = c.get(Calendar.DAY_OF_MONTH);
        int last = c.getActualMaximum(Calendar.DAY_OF_MONTH);
        return last - today;
    }

  public static void main(String[] args) {
        Integer days = monthEndNum("2023-01-20");
        System.out.println("2023年1月剩余天数:"+days);
    }

The test example returns results:

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