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

Question

I need to increment the month of a datetime value  next month   datetime datetime mydate year  mydate month 1  1    when the month is 12  it becomes 13 and raises error  month must be in 1  12    I expected the year would increment   I wanted to use timedelta  but it doesn t take month argument  There is relativedelta python package  but i don t want to install it just only for this  Also there is a solution using strtotime   time   strtotime str mydate    next month   date  Y-m-d   strtotime   1 month   time      I don t want to convert from datetime to str then to time  and then to datetime  therefore  it s still a library too  Does anyone have any good and simple solution just like using timedelta

User · Answer

What about this one   doesn t require any extra libraries   from datetime import date  timedelta from calendar import monthrange  today   date today   month later   date today year  today month  monthrange today year  today month  1     timedelta 1

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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

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To calculate the current  previous and next month   import datetime this month   datetime date today   month last month   datetime date today   month - 1 or 12 next month    datetime date today   month   1    12 or 12

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example using the time object   start time   time gmtime time time         start now   increment one month start time   time gmtime time mktime  start time tm year  start time tm mon 1  start time tm mday  start time tm hour  start time tm min  start time tm sec  0  0  0

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I was looking to solve the related problem of finding the date for the first of the following month  regardless of the day in the given date  This does not find the same day 1 month later   So  if all you want is to put in December 12  2014  or any day in December  and get back January 1  2015  try this   import datetime  def get next month date       month    date month   12    1     year   date year    date month   1  gt  12      return datetime datetime year  month  1

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A solution without the use of calendar   def add month year date  years 0  months 0       year  month   date year   years  date month   months   1     dyear  month   divmod month - 1  12      rdate   datetime date year   dyear  month   1  1  - datetime timedelta 1      return rdate replace day   min rdate day  date day

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Similar in ideal to Dave Webb s solution  but without all of that tricky modulo arithmetic   import datetime  calendar  def increment month date         Go to first of this month  and add 32 days to get to the next month     next month   date replace day 1    datetime timedelta 32        Get the day of month that corresponds     day   min date day  calendar monthrange next month year  next month month  1       return next month replace day day

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Simplest solution is to go at the end of the month  we always know that months have at least 28 days  and add enough days to move to the next moth    gt  gt  gt  from datetime import datetime  timedelta  gt  gt  gt  today   datetime today    gt  gt  gt  today datetime datetime 2014  4  30  11  47  27  811253   gt  gt  gt   today replace day 28    timedelta days 10   replace day today day  datetime datetime 2014  5  30  11  47  27  811253    Also works between years    gt  gt  gt  dec31 datetime datetime 2015  12  31  11  47  27  811253   gt  gt  gt  today   dec31  gt  gt  gt   today replace day 28    timedelta days 10   replace day today day  datetime datetime 2016  1  31  11  47  27  811253    Just keep in mind that it is not guaranteed that the next month will have the same day  for example when moving from 31 Jan to 31 Feb it will fail    gt  gt  gt  today datetime datetime 2016  1  31  11  47  27  811253   gt  gt  gt   today replace day 28    timedelta days 10   replace day today day  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  ValueError  day is out of range for month   So this is a valid solution if you need to move to the first day of the next month  as you always know that the next month has day 1   replace day 1    Otherwise  to move to the last available day  you might want to use    gt  gt  gt  today datetime datetime 2016  1  31  11  47  27  811253   gt  gt  gt  next month    today replace day 28    timedelta days 10    gt  gt  gt  import calendar  gt  gt  gt  next month replace day min today day                                  calendar monthrange next month year  next month month  1    datetime datetime 2016  2  29  11  47  27  811253

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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     Output      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 requires python-dateutil  To install it you need to run in Linux terminal   sudo apt-get update  amp  amp  sudo apt-get install python-dateutil   Explanation   Add month value in python

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This implementation might have some value for someone who is working with billing    If you are working with billing  you probably want to get  the same date next month  if possible   as opposed to  add 1 12 of one year    What is so confusing about this is you actually need take into account two values if you are doing this continuously  Otherwise for any dates past the 27th  you ll keep losing a few days until you end up at the 27th after leap year    The values you need to account for    The value you want to add a month to The day you started with   This way if you get bumped from the 31st down to the 30th when you add one month  you ll get bumped back up to the 31st for the next month that has that day   This is how I did it   def closest date next month year  month  day       month   month   1     if month    13          month   1         year    year   1       condition   True     while condition          try              return datetime datetime year  month  day          except ValueError              day   day-1         condition   day  gt  26      raise Exception  Problem getting date next month    paid until   closest date next month                   last paid until year                    last paid until month                    original purchase date day     The trick is here  I m using the original date  that I started adding from  not the last one

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from datetime import timedelta try      next    x replace day 1    timedelta days 31   replace day x day  except ValueError     January 31 will return last day of February      next    x   timedelta days 31   replace day 1  - timedelta days 1    If you simply want the first day of the next month   next    x replace day 1    timedelta days 31   replace day 1

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def add month d n 1   return type d  d year  d month n-1  12   d month n-1  12 1  1

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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

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Well with some tweaks and use of timedelta here we go    from datetime import datetime  timedelta   def inc date origin date       day   origin date day     month   origin date month     year   origin date year     if origin date month    12          delta   datetime year   1  1  day  - origin date     else          delta   datetime year  month   1  day  - origin date     return origin date   delta  final date   inc date datetime today    print final date date

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My very simple solution  which doesn t require any additional modules   def addmonth date       if date day  lt  20          date2   date timedelta 32      else           date2   date timedelta 25      date2 replace date2 year  date2 month  day      return date2

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This is what I came up with  from calendar  import monthrange  def same day months after start date  months 1       target year   start date year     start date month   months    12      target month    start date month   months    12     num days target month   monthrange target year  target month  1      return start date replace year target year  month target month           day min start date day  num days target month

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Just Use This   import datetime today   datetime datetime today   nextMonthDatetime   today   datetime timedelta days  today max day - today day  1

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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

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Perhaps add the number of days in the current month using calendar monthrange     import calendar  datetime  def increment month when       days   calendar monthrange when year  when month  1      return when   datetime timedelta days days   now   datetime datetime now   print  It is now  s    now print  In a month  it will be  s    increment month now

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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    gt  gt  gt  somedate   datetime date today    gt  gt  gt  somedate datetime date 2010  11  9   gt  gt  gt  add months somedate 1  datetime date 2010  12  9   gt  gt  gt  add months somedate 23  datetime date 2012  10  9   gt  gt  gt  otherdate   datetime date 2010 10 31   gt  gt  gt  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

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def month sub year  month  sub month       result month   0     result year   0     if month  gt   sub month   12           result month   month -  sub month   12          result year   year -  sub month   12      else          result month   12 -  sub month   12    month         result year   year -  sub month   12   1      return  result year  result month   def month add year  month  add month       return month sub year  month  -add month    gt  gt  gt  month add 2015  7  1                           2015  8   gt  gt  gt  month add 2015  7  20   2017  3   gt  gt  gt  month add 2015  7  12   2016  7   gt  gt  gt  month add 2015  7  24   2017  7   gt  gt  gt  month add 2015  7  -2   2015  5   gt  gt  gt  month add 2015  7  -12   2014  7   gt  gt  gt  month add 2015  7  -13   2014  6

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

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