问与答：我如何知道一个月的最后一天是哪一天？

Question

I was trying to write a roll-your-own timezone converter and I needed a way of determining what the last possible day of the month was. Upon some research, I discovered the formulas for finding a leap year.

It's a small contribution, but maybe I'll save someone else the 20 minutes it took me to figure out and apply it.

This code accepts a signed short month, indexed at 0 (0 is January) and an int year that is indexed as 0 as well (2012 is 2012).

It returns a 1 indexed day (the 27th is the 27th, but in SYSTEMTIME structures, etc., you usually need 0 indexed - just a head's up).

Answer 1

short _get_max_day(short month, int year) {
    if(month == 0 || month == 2 || month == 4 || month == 6 || month == 7 || month == 9 || month == 11)
        return 31;
    else if(month == 3 || month == 5 || month == 8 || month == 10)
        return 30;
    else {
        if(year % 4 == 0) {
            if(year % 100 == 0) {
                if(year % 400 == 0)
                    return 29;
                return 28;
            }
            return 29;
        }
        return 28;
    }
}

Answer 2

What about

#include <time.h>
#include <iostream>

int LastDay (int iMonth, int iYear)
{
    struct tm when;
    time_t lastday;

    // Set up current month
    when.tm_hour = 0;
    when.tm_min = 0;
    when.tm_sec = 0;
    when.tm_mday = 1;

    // Next month 0=Jan
    if (iMonth == 12)
    {
        when.tm_mon = 0;
        when.tm_year = iYear - 1900 + 1;
    }
    else
    {
        when.tm_mon = iMonth;
        when.tm_year = iYear - 1900;
    }
    // Get the first day of the next month
    lastday = mktime (&when);

    // Subtract 1 day
    lastday -= 86400;

    // Convert back to date and time
    when = *localtime (&lastday);

    return when.tm_mday;
}

int _tmain(int argc, _TCHAR* argv[])
{
    for (int m = 1; m <= 12; m++)
        std::cout << "Last day of " << m << " is " << LastDay (m, 2002) << std::endl;

    return 0;
}

It prints out (for year 2002)...

Last day of 1 is 31
Last day of 2 is 28
Last day of 3 is 31
Last day of 4 is 30
Last day of 5 is 31
Last day of 6 is 30
Last day of 7 is 31
Last day of 8 is 31
Last day of 9 is 30
Last day of 10 is 31
Last day of 11 is 30
Last day of 12 is 31

Answer 3

I use a simple function that returns the whole date in the from of a (Standard) COleDateTime. It may not be as fast other options, but it is very effective, works for leap years and pretty fool proof.

This is the Code that I am using:

COleDateTime get_last_day_of_month(UINT month, UINT year)    
{
if(month == 2)
    {                                               // if month is feb, take last day of March and then go back one day
        COleDateTime    date(year, 3, 1, 0, 0, 0);  // 1 March for Year
        date -= 1;                                  // go back one day (the standard class will take leap years into account)
        return date;
    }
    else if(month == 4 || month == 6 || month == 9 || month == 11) return COleDateTime(year, month, 30, 0, 0, 0);
    else return COleDateTime(year, month, 31, 0, 0, 0);
}

Answer 4

import datetime 
from datetime import date
from dateutil.relativedelta import relativedelta

year = int((date.today()).strftime("%Y"))
month = list(range(1, 13, 1))
YearMonthDay = [(datetime.datetime(year, x, 1) + relativedelta(day=31)).strftime("%Y%m%d") for x in month]
print(YearMonthDay)

['20220131', '20220228', '20220331', '20220430', '20220531', '20220630', '20220731', '20220831', '20220930', '20221031', '20221130', '20221231']

Answer 5

In C++20:

#include <chrono>

std::chrono::day
get_max_day(std::chrono::month m, std::chrono::year y)
{
    return (y/m/std::chrono::last).day();
}

If you really need it with a type-unsafe API:

int
get_max_day(int m, int y)
{
    return unsigned{(std::chrono::last/m/y).day()};
}

Answer 6

Word Year, Month, Day;
TDateTime datum_tdatetime = Date();

// first day of actual month
datum_tdatetime.DecodeDate(&year, &month, &day);
day = 1;
datum_tdatetime = EncodeDate(year, month, day);
// last day of previous month
datum_tdatetime -= 1;
// first day of previous month
datum_tdatetime.DecodeDate(&year, &month, &day);
day = 1;
datum_tdatetime = EncodeDate(year, month, day);