如何获得每月的最后一天？

Is there a way using Python’s standard library to easily determine (i.e. one function call) the last day of a given month?

If the standard library doesn’t support that, does the dateutil package support this?

I didn’t notice this earlier when I was looking at the documentation for the calendar module, but a method called monthrange provides this information:

monthrange(year, month)
Returns weekday of first day of the month and number of days in month, for the specified year and month.

>>> import calendar
>>> calendar.monthrange(2002,1)
(1, 31)
>>> calendar.monthrange(2008,2)
(4, 29)
>>> calendar.monthrange(2100,2)
(0, 28)

so:

calendar.monthrange(year, month)[1]

seems like the simplest way to go.

Just to be clear, monthrange supports leap years as well:

>>> from calendar import monthrange
>>> monthrange(2012, 2)
(2, 29)

My previous answer still works, but is clearly suboptimal.

If you don’t want to import the calendar module, a simple two-step function can also be:

import datetime

def last_day_of_month(any_day):
    next_month = any_day.replace(day=28) + datetime.timedelta(days=4)  # this will never fail
    return next_month - datetime.timedelta(days=next_month.day)

Outputs:

>>> for month in range(1, 13):
...     print last_day_of_month(datetime.date(2012, month, 1))
...
2012-01-31
2012-02-29
2012-03-31
2012-04-30
2012-05-31
2012-06-30
2012-07-31
2012-08-31
2012-09-30
2012-10-31
2012-11-30
2012-12-31

EDIT: See @Blair Conrad’s answer for a cleaner solution

>>> import datetime
>>> datetime.date(2000, 2, 1) - datetime.timedelta(days=1)
datetime.date(2000, 1, 31)

This is actually pretty easy with dateutil.relativedelta (package python-datetutil for pip). day=31 will always always return the last day of the month.

Example:

from datetime import datetime
from dateutil.relativedelta import relativedelta

date_in_feb = datetime.datetime(2013, 2, 21)
print datetime.datetime(2013, 2, 21) + relativedelta(day=31)  # End-of-month
>>> datetime.datetime(2013, 2, 28, 0, 0)

EDIT: see my other answer. It has a better implementation than this one, which I leave here just in case someone’s interested in seeing how one might “roll your own” calculator.

@John Millikin gives a good answer, with the added complication of calculating the first day of the next month.

The following isn’t particularly elegant, but to figure out the last day of the month that any given date lives in, you could try:

def last_day_of_month(date):
    if date.month == 12:
        return date.replace(day=31)
    return date.replace(month=date.month+1, day=1) - datetime.timedelta(days=1)

>>> last_day_of_month(datetime.date(2002, 1, 17))
datetime.date(2002, 1, 31)
>>> last_day_of_month(datetime.date(2002, 12, 9))
datetime.date(2002, 12, 31)
>>> last_day_of_month(datetime.date(2008, 2, 14))
datetime.date(2008, 2, 29)

Using dateutil.relativedelta you would get last date of month like this:

from dateutil.relativedelta import relativedelta
last_date_of_month = datetime(mydate.year, mydate.month, 1) + relativedelta(months=1, days=-1)

The idea is to get the first day of the month and use relativedelta to go 1 month ahead and 1 day back so you would get the last day of the month you wanted.

Another solution would be to do something like this:

from datetime import datetime

def last_day_of_month(year, month):
    """ Work out the last day of the month """
    last_days = [31, 30, 29, 28, 27]
    for i in last_days:
        try:
            end = datetime(year, month, i)
        except ValueError:
            continue
        else:
            return end.date()
    return None

And use the function like this:

>>> 
>>> last_day_of_month(2008, 2)
datetime.date(2008, 2, 29)
>>> last_day_of_month(2009, 2)
datetime.date(2009, 2, 28)
>>> last_day_of_month(2008, 11)
datetime.date(2008, 11, 30)
>>> last_day_of_month(2008, 12)
datetime.date(2008, 12, 31)

from datetime import timedelta
(any_day.replace(day=1) + timedelta(days=32)).replace(day=1) - timedelta(days=1)

>>> import datetime
>>> import calendar
>>> date  = datetime.datetime.now()

>>> print date
2015-03-06 01:25:14.939574

>>> print date.replace(day = 1)
2015-03-01 01:25:14.939574

>>> print date.replace(day = calendar.monthrange(date.year, date.month)[1])
2015-03-31 01:25:14.939574

if you are willing to use an external library, check out http://crsmithdev.com/arrow/

U can then get the last day of the month with:

import arrow
arrow.utcnow().ceil('month').date()

This returns a date object which you can then do your manipulation.

To get the last date of the month we do something like this:

from datetime import date, timedelta
import calendar
last_day = date.today().replace(day=calendar.monthrange(date.today().year, date.today().month)[1])

Now to explain what we are doing here we will break it into two parts:

first is getting the number of days of the current month for which we use monthrange which Blair Conrad has already mentioned his solution:

calendar.monthrange(date.today().year, date.today().month)[1]

second is getting the last date itself which we do with the help of replace e.g

>>> date.today()
datetime.date(2017, 1, 3)
>>> date.today().replace(day=31)
datetime.date(2017, 1, 31)

and when we combine them as mentioned on the top we get a dynamic solution.

In Python 3.7 there is the undocumented calendar.monthlen(year, month) function:

>>> calendar.monthlen(2002, 1)
31
>>> calendar.monthlen(2008, 2)
29
>>> calendar.monthlen(2100, 2)
28

It is equivalent to the documented calendar.monthrange(year, month)[1] call.

import datetime

now = datetime.datetime.now()
start_month = datetime.datetime(now.year, now.month, 1)
date_on_next_month = start_month + datetime.timedelta(35)
start_next_month = datetime.datetime(date_on_next_month.year, date_on_next_month.month, 1)
last_day_month = start_next_month - datetime.timedelta(1)

Use pandas!

def isMonthEnd(date):
    return date + pd.offsets.MonthEnd(0) == date

isMonthEnd(datetime(1999, 12, 31))
True
isMonthEnd(pd.Timestamp('1999-12-31'))
True
isMonthEnd(pd.Timestamp(1965, 1, 10))
False

For me it’s the simplest way:

selected_date = date(some_year, some_month, some_day)

if selected_date.month == 12: # December
     last_day_selected_month = date(selected_date.year, selected_date.month, 31)
else:
     last_day_selected_month = date(selected_date.year, selected_date.month + 1, 1) - timedelta(days=1)

The easiest way (without having to import calendar), is to get the first day of the next month, and then subtract a day from it.

import datetime as dt
from dateutil.relativedelta import relativedelta

thisDate = dt.datetime(2017, 11, 17)

last_day_of_the_month = dt.datetime(thisDate.year, (thisDate + relativedelta(months=1)).month, 1) - dt.timedelta(days=1)
print last_day_of_the_month

Output:

datetime.datetime(2017, 11, 30, 0, 0)

PS: This code runs faster as compared to the import calendarapproach; see below:

import datetime as dt
import calendar
from dateutil.relativedelta import relativedelta

someDates = [dt.datetime.today() - dt.timedelta(days=x) for x in range(0, 10000)]

start1 = dt.datetime.now()
for thisDate in someDates:
    lastDay = dt.datetime(thisDate.year, (thisDate + relativedelta(months=1)).month, 1) - dt.timedelta(days=1)

print ('Time Spent= ', dt.datetime.now() - start1)


start2 = dt.datetime.now()
for thisDate in someDates:
    lastDay = dt.datetime(thisDate.year, 
                          thisDate.month, 
                          calendar.monthrange(thisDate.year, thisDate.month)[1])

print ('Time Spent= ', dt.datetime.now() - start2)

OUTPUT:

Time Spent=  0:00:00.097814
Time Spent=  0:00:00.109791

This code assumes that you want the date of the last day of the month (i.e., not just the DD part, but the entire YYYYMMDD date)

Here is another answer. No extra packages required.

datetime.date(year + int(month/12), month%12+1, 1)-datetime.timedelta(days=1)

Get the first day of the next month and subtract a day from it.

You can calculate the end date yourself. the simple logic is to subtract a day from the start_date of next month. :)

So write a custom method,

import datetime

def end_date_of_a_month(date):


    start_date_of_this_month = date.replace(day=1)

    month = start_date_of_this_month.month
    year = start_date_of_this_month.year
    if month == 12:
        month = 1
        year += 1
    else:
        month += 1
    next_month_start_date = start_date_of_this_month.replace(month=month, year=year)

    this_month_end_date = next_month_start_date - datetime.timedelta(days=1)
    return this_month_end_date

Calling,

end_date_of_a_month(datetime.datetime.now().date())

It will return the end date of this month. Pass any date to this function. returns you the end date of that month.

you can use relativedelta https://dateutil.readthedocs.io/en/stable/relativedelta.html month_end = <your datetime value within the month> + relativedelta(day=31) that will give you the last day.

This is the simplest solution for me using just the standard datetime library:

import datetime

def get_month_end(dt):
    first_of_month = datetime.datetime(dt.year, dt.month, 1)
    next_month_date = first_of_month + datetime.timedelta(days=32)
    new_dt = datetime.datetime(next_month_date.year, next_month_date.month, 1)
    return new_dt - datetime.timedelta(days=1)

This does not address the main question, but one nice trick to get the last weekday in a month is to use calendar.monthcalendar, which returns a matrix of dates, organized with Monday as the first column through Sunday as the last.

# Some random date.
some_date = datetime.date(2012, 5, 23)

# Get last weekday
last_weekday = np.asarray(calendar.monthcalendar(some_date.year, some_date.month))[:,0:-2].ravel().max()

print last_weekday
31

The whole [0:-2] thing is to shave off the weekend columns and throw them out. Dates that fall outside of the month are indicated by 0, so the max effectively ignores them.

The use of numpy.ravel is not strictly necessary, but I hate relying on the mere convention that numpy.ndarray.max will flatten the array if not told which axis to calculate over.

import calendar
from time import gmtime, strftime
calendar.monthrange(int(strftime("%Y", gmtime())), int(strftime("%m", gmtime())))[1]

Output:

31

This will print the last day of whatever the current month is. In this example it was 15th May, 2016. So your output may be different, however the output will be as many days that the current month is. Great if you want to check the last day of the month by running a daily cron job.

So:

import calendar
from time import gmtime, strftime
lastDay = calendar.monthrange(int(strftime("%Y", gmtime())), int(strftime("%m", gmtime())))[1]
today = strftime("%d", gmtime())
lastDay == today

Output:

False

Unless it IS the last day of the month.

I prefer this way

import datetime
import calendar

date=datetime.datetime.now()
month_end_date=datetime.datetime(date.year,date.month,1) + datetime.timedelta(days=calendar.monthrange(date.year,date.month)[1] - 1)

If you want to make your own small function, this is a good starting point:

def eomday(year, month):
    """returns the number of days in a given month"""
    days_per_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    d = days_per_month[month - 1]
    if month == 2 and (year % 4 == 0 and year % 100 != 0 or year % 400 == 0):
        d = 29
    return d

For this you have to know the rules for the leap years:

• every fourth year
• with the exception of every 100 year
• but again every 400 years

If you pass in a date range, you can use this:

def last_day_of_month(any_days):
    res = []
    for any_day in any_days:
        nday = any_day.days_in_month -any_day.day
        res.append(any_day + timedelta(days=nday))
    return res

In the code below ‘get_last_day_of_month(dt)’ will give you this, with date in string format like ‘YYYY-MM-DD’.

import datetime

def DateTime( d ):
    return datetime.datetime.strptime( d, '%Y-%m-%d').date()

def RelativeDate( start, num_days ):
    d = DateTime( start )
    return str( d + datetime.timedelta( days = num_days ) )

def get_first_day_of_month( dt ):
    return dt[:-2] + '01'

def get_last_day_of_month( dt ):
    fd = get_first_day_of_month( dt )
    fd_next_month = get_first_day_of_month( RelativeDate( fd, 31 ) )
    return RelativeDate( fd_next_month, -1 )

The simplest way is to use datetime and some date math, e.g. subtract a day from the first day of the next month:

import datetime

def last_day_of_month(d: datetime.date) -> datetime.date:
    return (
        datetime.date(d.year + d.month//12, d.month % 12 + 1, 1) -
        datetime.timedelta(days=1)
    )

Alternatively, you could use calendar.monthrange() to get the number of days in a month (taking leap years into account) and update the date accordingly:

import calendar, datetime

def last_day_of_month(d: datetime.date) -> datetime.date:
    return d.replace(day=calendar.monthrange(d.year, d.month)[1])

A quick benchmark shows that the first version is noticeably faster:

In [14]: today = datetime.date.today()

In [15]: %timeit last_day_of_month_dt(today)
918 ns ± 3.54 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [16]: %timeit last_day_of_month_calendar(today)
1.4 µs ± 17.3 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

Here is a long (easy to understand) version but takes care of leap years.

cheers, JK

def last_day_month(year, month):
    leap_year_flag = 0
    end_dates = {
        1: 31,
        2: 28,
        3: 31,
        4: 30,
        5: 31,
        6: 30,
        7: 31,
        8: 31,
        9: 30,
        10: 31,
        11: 30,
        12: 31
    }

    # Checking for regular leap year    
    if year % 4 == 0:
        leap_year_flag = 1
    else:
        leap_year_flag = 0

    # Checking for century leap year    
    if year % 100 == 0:
        if year % 400 == 0:
            leap_year_flag = 1
        else:
            leap_year_flag = 0
    else:
        pass

    # return end date of the year-month
    if leap_year_flag == 1 and month == 2:
        return 29
    elif leap_year_flag == 1 and month != 2:
        return end_dates[month]
    else:
        return end_dates[month]

Here is a solution based python lambdas:

next_month = lambda y, m, d: (y, m + 1, 1) if m + 1 < 13 else ( y+1 , 1, 1)
month_end  = lambda dte: date( *next_month( *dte.timetuple()[:3] ) ) - timedelta(days=1)

The next_month lambda finds the tuple representation of the first day of the next month, and rolls over to the next year. The month_end lambda transforms a date (dte) to a tuple, applies next_month and creates a new date. Then the “month’s end” is just the next month’s first day minus timedelta(days=1).