How to increment datetime by custom months in python without using library [duplicate]

Question

This is short and sweet method to add a month to a date using dateutil's relativedelta.

from datetime import datetime
from dateutil.relativedelta import relativedelta
    
date_after_month = datetime.today()+ relativedelta(months=1)
print 'Today: ',datetime.today().strftime('%d/%m/%Y')
print 'After Month:', date_after_month.strftime('%d/%m/%Y')

Today:  01/03/2013

After Month: 01/04/2013

A word of warning: relativedelta(months=1) and relativedelta(month=1) have different meanings. Passing month=1 will replace the month in original date to January whereas passing months=1 will add one month to original date.

Note: this will require python-dateutil module. If you are on Linux you need to run this command in the terminal in order to install it.

sudo apt-get update && sudo apt-get install python-dateutil

Explanation : Add month value in python

Edit - based on your comment of dates being needed to be rounded down if there are fewer days in the next month, here is a solution:

import datetime
import calendar

def add_months(sourcedate, months):
    month = sourcedate.month - 1 + months
    year = sourcedate.year + month // 12
    month = month % 12 + 1
    day = min(sourcedate.day, calendar.monthrange(year,month)[1])
    return datetime.date(year, month, day)

In use:

>>> somedate = datetime.date.today()
>>> somedate
datetime.date(2010, 11, 9)
>>> add_months(somedate,1)
datetime.date(2010, 12, 9)
>>> add_months(somedate,23)
datetime.date(2012, 10, 9)
>>> otherdate = datetime.date(2010,10,31)
>>> add_months(otherdate,1)
datetime.date(2010, 11, 30)

Also, if you're not worried about hours, minutes and seconds you could use date rather than datetime. If you are worried about hours, minutes and seconds you need to modify my code to use datetime and copy hours, minutes and seconds from the source to the result.

Here's my salt :

current = datetime.datetime(mydate.year, mydate.month, 1)
next_month = datetime.datetime(mydate.year + int(mydate.month / 12), ((mydate.month % 12) + 1), 1)

Quick and easy :)

since no one suggested any solution, here is how i solved so far

year, month= divmod(mydate.month+1, 12)
if month == 0: 
      month = 12
      year = year -1
next_month = datetime.datetime(mydate.year + year, month, 1)

Use the monthdelta package, it works just like timedelta but for calendar months rather than days/hours/etc.

Here's an example:

from monthdelta import MonthDelta

def prev_month(date):
    """Back one month and preserve day if possible"""
    return date + MonthDelta(-1)

Compare that to the DIY approach:

def prev_month(date):
    """Back one month and preserve day if possible"""
   day_of_month = date.day
   if day_of_month != 1:
           date = date.replace(day=1)
   date -= datetime.timedelta(days=1)
   while True:
           try:
                   date = date.replace(day=day_of_month)
                   return date
           except ValueError:
                   day_of_month -= 1