Best way to find the months between two dates

Question

I have the need to be able to accurately find the months between two dates in python  I have a solution that works but its not very good  as in elegant  or fast    dateRange    datetime strptime dateRanges 0     Y- m- d    datetime strptime dateRanges 1     Y- m- d    months        tmpTime   dateRange 0  oneWeek   timedelta weeks 1  tmpTime   tmpTime replace day 1  dateRange 0    tmpTime dateRange 1    dateRange 1  replace day 1  lastMonth   tmpTime month months append tmpTime  while tmpTime  lt  dateRange 1       if lastMonth    12          while tmpTime month  lt   lastMonth              tmpTime    oneWeek         tmpTime   tmpTime replace day 1          months append tmpTime          lastMonth   tmpTime month      else          while tmpTime month  gt   lastMonth              tmpTime    oneWeek         tmpTime   tmpTime replace day 1          months append tmpTime          lastMonth   tmpTime month   So just to explain  what I m doing here is taking the two dates and converting them from iso format into python datetime objects  Then I loop through adding a week to the start datetime object and check if the numerical value of the month is greater  unless the month is December then it checks if the date is less   If the value is greater I append it to the list of months and keep looping through until I get to my end date   It works perfectly it just doesn t seem like a good way of doing it

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Start by defining some test cases  then you will see that the function is very simple and needs no loops  from datetime import datetime  def diff month d1  d2       return  d1 year - d2 year    12   d1 month - d2 month  assert diff month datetime 2010 10 1   datetime 2010 9 1      1 assert diff month datetime 2010 10 1   datetime 2009 10 1      12 assert diff month datetime 2010 10 1   datetime 2009 11 1      11 assert diff month datetime 2010 10 1   datetime 2009 8 1      14   You should add some test cases to your question  as there are lots of potential corner cases to cover - there is more than one way to define the number of months between two dates

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just like range function  when month is 13  go to next year  def year month range start date  end date               start date  datetime date 2015  9  1  or datetime datetime     end date  datetime date 2016  3  1  or datetime datetime     return  datetime date list of 201509  201510  201511  201512  201601  201602             start  end   start date strftime   Y m    end date strftime   Y m       assert len start     6 and len end     6     start  end   int start   int end       year month list          while start  lt  end          year  month   divmod start  100          if month    13              start    88    201513   88   201601             continue         year month list append datetime date year  month  1            start    1     return year month list   example in python shell   gt  gt  gt  import datetime  gt  gt  gt  s   datetime date 2015 9 1   gt  gt  gt  e   datetime date 2016  3  1   gt  gt  gt  year month set range s  e   datetime date 2015  11  1   datetime date 2015  9  1   datetime date 2016  1  1   datetime date 2016  2  1    datetime date 2015  12  1   datetime date 2015  10  1

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This is my way to do this   Start date    2000-06-01  End date      2001-05-01   month num    len pd date range start   Start date  7   end   End date  7   freq  M    1   I just use the month to create a date range and calculate the length

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from dateutil import relativedelta  relativedelta relativedelta date1  date2   months difference    r years   12    r months

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Try this    dateRange    datetime strptime dateRanges 0     Y- m- d                 datetime strptime dateRanges 1     Y- m- d    delta time   max dateRange  - min dateRange   Need to use min dateRange  month to account for different length month  Note that timedelta returns a number of days delta datetime    datetime 1  min dateRange  month  1    delta time -                            timedelta days 1    min y m d are 1 months     delta datetime year - 1    12   delta datetime month -           min dateRange  month  print months   Shouldn t matter what order you input the dates  and it takes into account the difference in month lengths

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This post nails it  Use dateutil relativedelta   from datetime import datetime from dateutil import relativedelta date1   datetime strptime str  2011-08-15 12 00 00      Y- m- d  H  M  S   date2   datetime strptime str  2012-02-15      Y- m- d   r   relativedelta relativedelta date2  date1  r months

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You can use the below code to get month between two dates   OrderedDict   start date   timedelta     strftime date format   None  for   in xrange  end date - start date  days   keys     where start date and end date must be proper date and date format is the format in which you want your result of date    In your case  date format will be  b  Y

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You can easily calculate this using rrule from dateutil module   from dateutil import rrule from datetime import date  print list rrule rrule rrule MONTHLY  dtstart date 2013  11  1   until date 2014  2  1       will give you     datetime datetime 2013  11  1  0  0    datetime datetime 2013  12  1  0  0    datetime datetime 2014  1  1  0  0    datetime datetime 2014  2  1  0  0

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Here s how to do this with Pandas FWIW   import pandas as pd pd date range  1990 04 03    2014 12 31   freq  MS    DatetimeIndex   1990-05-01    1990-06-01    1990-07-01    1990-08-01                   1990-09-01    1990-10-01    1990-11-01    1990-12-01                   1991-01-01    1991-02-01                                      2014-03-01    2014-04-01    2014-05-01    2014-06-01                   2014-07-01    2014-08-01    2014-09-01    2014-10-01                   2014-11-01    2014-12-01                  dtype  datetime64 ns    length 296  freq  MS     Notice it starts with the month after the given start date

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Get the ending month  relative to the year and month of the start month ex  2011 January   13 if your start date starts on 2010 Oct  and then generate the datetimes beginning the start month and that end month like so   dt1  dt2   dateRange start month dt1 month end months  dt2 year-dt1 year  12   dt2 month 1 dates  datetime datetime year yr  month mn  day 1  for  yr  mn  in               m - 1    12   dt1 year   m - 1    12   1  for m in range start month  end months             if both dates are on the same year  it could also be simply written as   dates  datetime datetime year dt1 year  month mn  day 1  for mn in range dt1 month  dt2 month   1

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Update 2018-04-20  it seems that OP  Joshkunz was asking for finding which months are between two dates  instead of  how many months  are between two dates  So I am not sure why  JohnLaRooy is upvoted for more than 100 times   Joshkunz indicated in the comment under the original question he wanted the actual dates  or the months   instead of finding the total number of months   So it appeared the question wanted  for between two dates 2018-04-11 to 2018-06-01  Apr 2018  May 2018  June 2018    And what if it is between 2014-04-11 to 2018-06-01  Then the answer would be   Apr 2014  May 2014       Dec 2014  Jan 2015       Jan 2018       June 2018   So that s why I had the following pseudo code many years ago  It merely suggested using the two months as end points and loop through them  incrementing by one month at a time   Joshkunz mentioned he wanted the  months  and he also mentioned he wanted the  dates   without knowing exactly  it was difficult to write the exact code  but the idea is to use one simple loop to loop through the end points  and incrementing one month at a time   The answer 8 years ago in 2010   If adding by a week  then it will approximately do work 4 35 times the work as needed   Why not just   1  get start date in array of integer  set it to i   2008  3  12           and change it to  2008  3  1  2  get end date in array   2010  10  26  3  add the date to your result by parsing i        increment the month in i        if month is  gt   13  then set it to 1  and increment the year by 1    until either the year in i is  gt  year in end date              or  year in i    year in end date and month in i  gt  month in end date    just pseduo code for now  haven t tested  but i think the idea along the same line will work

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You could use something like   import datetime days in month   365 25   12    represent the average of days in a month by year month diff   lambda end date  start date  precision 0  round  end date - start date  days   days in month  precision  start date   datetime date 1978  12  15  end date   datetime date 2012  7  9  month diff end date  start date     should show 403 0 months

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There is a simple solution based on 360 day years  where all months have 30 days  It fits most use cases where  given two dates  you need to calculate the number of full months plus the remaining days   from datetime import datetime  timedelta  def months between start date  end date        Add 1 day to end date to solve different last days of month      s1  e1   start date   end date    timedelta days 1       Convert to 360 days     s360    s1 year   12   s1 month    30   s1 day     e360    e1 year   12   e1 month    30   e1 day      Count days between the two 360 dates and return tuple  months  days      return divmod e360 - s360  30   print  Counting full and half months  print months between  datetime 2012  01  1   datetime 2012  03  31    3m print months between  datetime 2012  01  1   datetime 2012  03  15    2m 15d print months between  datetime 2012  01  16   datetime 2012  03  31    2m 15d print months between  datetime 2012  01  16   datetime 2012  03  15    2m print  Adding  1d and -1d to 31 day month  print months between  datetime 2011  12  01   datetime 2011  12  31    1m 0d print months between  datetime 2011  12  02   datetime 2011  12  31    -1d   gt  29d print months between  datetime 2011  12  01   datetime 2011  12  30    30d   gt  1m print  Adding  1d and -1d to 29 day month  print months between  datetime 2012  02  01   datetime 2012  02  29    1m 0d print months between  datetime 2012  02  02   datetime 2012  02  29    -1d   gt  29d print months between  datetime 2012  02  01   datetime 2012  02  28    28d print  Every month has 30 days - 26 M to 5 M 1 always counts 10 days  print months between  datetime 2011  02  26   datetime 2011  03  05   print months between  datetime 2012  02  26   datetime 2012  03  05   print months between  datetime 2012  03  26   datetime 2012  04  05

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This works     from datetime import datetime as dt from dateutil relativedelta import relativedelta def number of months d1  d2       months   0     r   relativedelta d1 d2      if r years  0          months   r months     if r years gt  1          months   12 r years r months     return months  example  number of months dt 2017 9 1  dt 2016 8 1

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Assuming upperDate is always later than lowerDate and both are datetime date objects   if lowerDate year    upperDate year      monthsInBetween   range  lowerDate month   1  upperDate month   elif upperDate year  gt  lowerDate year      monthsInBetween   range  lowerDate month   1  12       for year in range  lowerDate year   1  upperDate year            monthsInBetween extend  range 1 13        monthsInBetween extend  range  1  upperDate month       I haven t tested this thoroughly  but it looks like it should do the trick

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from datetime import datetime from dateutil import relativedelta  def get months d1  d2       date1   datetime strptime str d1     Y- m- d       date2   datetime strptime str d2     Y- m- d       print  date2  date1      r   relativedelta relativedelta date2  date1      months   r months    12   r years     if r days  gt  0          months    1     print  months      return  months   assert  get months  2018-08-13   2019-06-19      11 assert  get months  2018-01-01   2019-06-19      18 assert  get months  2018-07-20   2019-06-19      11 assert  get months  2018-07-18   2019-06-19      12 assert  get months  2019-03-01   2019-06-19      4 assert  get months  2019-03-20   2019-06-19      3 assert  get months  2019-01-01   2019-06-19      6 assert  get months  2018-09-09   2019-06-19      10

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Usually 90 days are NOT 3 months literally  just a reference   So  finally  you need to check if days are bigger than 15 to add  1 to month counter  or better  add another elif with half month counter   From this other stackoverflow answer i ve finally ended with that     usr bin env python   - - coding  utf8 - -  import datetime from datetime import timedelta from dateutil relativedelta import relativedelta import calendar  start date   datetime date today   end date   start date   timedelta days 111  start month   calendar month abbr int start date strftime   m      print str start date      to     str end date   months   relativedelta end date  start date  months days   relativedelta end date  start date  days  print months   months   days   days   if days  gt  16      months    1  print  around     str months      months         for i in range 0  months       print calendar month abbr int start date strftime   m          start date   start date   relativedelta months 1   print       Output   2016-02-29 2016-06-14 3 months 16 days around 4 months   Feb Mar Apr May     I ve noticed that doesn t work if you add more than days left in current year  and that s is unexpected

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Here is my solution for this   def calc age months from date  to date       from date   time strptime from date    Y- m- d       to date   time strptime to date    Y- m- d        age in months    to date tm year - from date tm year  12    to date tm mon - from date tm mon       if to date tm mday  lt  from date tm mday          return age in months -1     else         return age in months   This will handle some edge cases as well where the difference in months between 31st Dec 2018 and 1st Jan 2019 will be zero  since the difference is only a day

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It can be done using datetime timedelta  where the number of days for skipping to next month can be obtained by calender monthrange  monthrange returns weekday  0-6   Mon-Sun  and number of days  28-31  for a given year and month  For example  monthrange 2017  1  returns  6 31    Here is the script using this logic to iterate between two months     from datetime import timedelta import datetime as dt from calendar import monthrange  def month iterator start month  end month       start month   dt datetime strptime start month                                       Y- m- d   date   replace day 1      end month   dt datetime strptime end month                                     Y- m- d   date   replace day 1      while start month  lt   end month          yield start month         start month   start month   timedelta days monthrange start month year                                                            start month month  1

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from datetime import datetime  def diff month start date end date       qty month     end date year - start date year    12     end date month - start date month       d days   end date day - start date day      if d days  gt   0          adjust   0     else          adjust   -1     qty month    adjust      return qty month  diff month datetime date today   datetime 2019 08 24      Examples   diff month datetime 2018 02 12  datetime 2019 08 24     18  diff month datetime 2018 02 12  datetime 2018 08 10     5

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To get the number of full months between two dates   import datetime  def difference in months start  end       if start year    end year          months   end month - start month     else          months    12 - start month     end month       if start day  gt  end day          months   months - 1      return months

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You can also use the arrow library  This is a simple example   from datetime import datetime import arrow  start   datetime 2014  1  17  end   datetime 2014  6  20   for d in arrow Arrow range  month   start  end       print d month  d format  MMMM     This will print   1 January 2 February 3 March 4 April 5 May 6 June   Hope this helps

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Many people have already given you good answers to solve this but I have not read any using list comprehension so I give you what I used for a similar use case     def compute months first date  second date       year1  month1  year2  month2   map          int            first date  4   first date 5 7   second date  4   second date 5 7              return              0 gt 4 -  0 gt 2   format year  month          for year in range year1  year2   1          for month in range month1 if year    year1 else 1  month2   1 if year    year2 else 13          gt  gt  gt  first date    2016-05   gt  gt  gt  second date    2017-11   gt  gt  gt  compute months first date  second date    2016-05     2016-06     2016-07     2016-08     2016-09     2016-10     2016-11     2016-12     2017-01     2017-02     2017-03     2017-04     2017-05     2017-06     2017-07     2017-08     2017-09     2017-10     2017-11

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import datetime from calendar import monthrange  def date dif from date to date                                            dd  to date-from date  days     if dd gt  0          fromDM from date year 12 from date month-1         toDM to date year 12 to date month-1         mlen monthrange int  toDM  12   toDM  12 1  1          d to date day-from date day         dm toDM-fromDM         m  dm-int d lt 0   12         y int  dm-int d lt 0   12          d  int d lt 0  mlen           diference in Y M D  diference months diference  days  days in to date month         return y m d dm dd mlen      else          return 0 0 0 0 dd 0

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You could use python-dateutil  See Python  Difference of 2 datetimes in months

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This definition gives an array of months between two dates  import datetime def MonthsBetweenDates BeginDate  EndDate       firstyearmonths    mn for mn in range BeginDate month  13   lt p gt      lastyearmonths    mn for mn in range 1  EndDate month 1   lt p gt      months    mn for mn in range 1  13   lt p gt      numberofyearsbetween   EndDate year - BeginDate year - 1 lt p gt      return firstyearmonths   months   numberofyearsbetween   lastyearmonths lt p gt    example BD   datetime datetime strptime  2000-35     Y- j   ED   datetime datetime strptime  2004-200     Y- j   MonthsBetweenDates BD  ED

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I actually needed to do something pretty similar just now  Ended up writing a function which returns a list of tuples indicating the start and end of each month between two sets of dates so I could write some SQL queries off the back of it for monthly totals of sales etc   I m sure it can be improved by someone who knows what they re doing but hope it helps     The returned value look as follows  generating for today - 365days until today as an example        datetime date 2013  5  1   datetime date 2013  5  31         datetime date 2013  6  1   datetime date 2013  6  30         datetime date 2013  7  1   datetime date 2013  7  31         datetime date 2013  8  1   datetime date 2013  8  31         datetime date 2013  9  1   datetime date 2013  9  30         datetime date 2013  10  1   datetime date 2013  10  31         datetime date 2013  11  1   datetime date 2013  11  30         datetime date 2013  12  1   datetime date 2013  12  31         datetime date 2014  1  1   datetime date 2014  1  31         datetime date 2014  2  1   datetime date 2014  2  28         datetime date 2014  3  1   datetime date 2014  3  31         datetime date 2014  4  1   datetime date 2014  4  30         datetime date 2014  5  1   datetime date 2014  5  31      Code as follows  has some debug stuff which can be removed        usr env python import datetime  def gen month ranges start date None  end date None  debug False       today   datetime date today       if not start date  start date   datetime datetime strptime            0  01 01  format today year    Y  m  d   date      start of this year     if not end date  end date   today     if debug  print  Start   0    End  1   format start date  end date          sense-check     if end date  lt  start date          print  Error  Start Date of  0  is greater than End Date of  1     format start date  end date           return None      date ranges         list of tuples  month start  month end       current year   start date year     current month   start date month      while current year  lt   end date year          next month   current month   1         next year   current year         if next month  gt  12              next month   1             next year   current year   1          month start   datetime datetime strptime                0   1  01  format current year                                  current month    Y  m  d   date      start of month         month end   datetime datetime strptime                0   1  01  format next year                                  next month    Y  m  d   date      start of next month         month end    month end datetime timedelta days -1     start of next month less one day          range tuple    month start  month end          if debug  print  Month runs from  0  -- gt   1   format              range tuple 0   range tuple 1            date ranges append range tuple           if current month    12              current month   1             current year    1             if debug  print  End of year encountered  resetting months           else              current month    1             if debug  print  Next iteration for  0 - 1   format                  current year  current month            if current year    end date year and current month  gt  end date month              if debug  print  Final month encountered  Terminating loop               break      return date ranges   if   name         main         print  Running in standalone mode  Debug set to True       from pprint import pprint     pprint gen month ranges debug True   indent 4      pprint gen month ranges start date datetime date today   datetime timedelta days -365                               debug True   indent 4

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Somewhat a little prettified solution by  Vin-G   import datetime  def monthrange start  finish     months    finish year - start year    12   finish month   1    for i in xrange start month  months       year     i - 1    12   start year      month    i - 1    12   1     yield datetime date year  month  1

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it seems that the answers are unsatisfactory and I have since use my own code which is easier to understand  from datetime import datetime from dateutil import relativedelta  date1   datetime strptime str  2017-01-01      Y- m- d   date2   datetime strptime str  2019-03-19      Y- m- d    difference   relativedelta relativedelta date2  date1  months   difference months years   difference years   add in the number of months  12  for difference in years months    12   difference years months

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One liner to find a list of datetimes  incremented by month  between two dates    import datetime from dateutil rrule import rrule  MONTHLY  strt dt   datetime date 2001 1 1  end dt   datetime date 2005 6 1   dates    dt for dt in rrule MONTHLY  dtstart strt dt  until end dt

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Here is a method   def months between start dt  stop dt       month list          total months   12  stop dt year-start dt year   stop dt month-start d month  1     if total months  gt  0          month list   datetime date start dt year int  start dt i-1  12                                         start dt-1 i  12  1                                     1  for i in xrange 0 total months        return month list   This is first computing the total number of months between the two dates  inclusive   Then it creates a list using the first date as the base and performs modula arithmetic to create the date objects

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My simple solution   import datetime  def months d1  d2       return d1 month - d2 month   12  d1 year - d2 year   d1   datetime datetime 2009  9  26    d2   datetime datetime 2019  9  26    print months d1  d2

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Assuming that you wanted to know the  fraction  of the month that dates were in  which I did  then you need to do a bit more work   from datetime import datetime  date import calendar  def monthdiff start period  end period  decimal places   2       if start period  gt  end period          raise Exception  Start is after end       if start period year    end period year and start period month    end period month          days in month   calendar monthrange start period year  start period month  1          days to charge   end period day - start period day 1         diff   round float days to charge  float days in month   decimal places          return diff     months   0       we have a start date within one month and not at the start  and an end date that is not       in the same month as the start date     if start period day  gt  1          last day in start month   calendar monthrange start period year  start period month  1          days to charge   last day in start month - start period day  1         months   months   round float days to charge  float last day in start month   decimal places          start period   datetime start period year  start period month 1  1       last day in last month   calendar monthrange end period year  end period month  1      if end period day    last day in last month            we have lest days in the last month         months   months   round float end period day    float last day in last month   decimal places          last day in previous month   calendar monthrange end period year  end period month - 1  1          end period   datetime end period year  end period month - 1  last day in previous month        whatever happens  we now have a period of whole months to calculate the difference between      if start period    end period          months   months    end period year - start period year    12    end period month - start period month    1        just counter for any final decimal place manipulation     diff   round months  decimal places      return diff  assert monthdiff datetime 2015 1 1   datetime 2015 1 31      1 assert monthdiff datetime 2015 1 1   datetime 2015 02 01      1 04 assert monthdiff datetime 2014 1 1   datetime 2014 12 31      12 assert monthdiff datetime 2014 7 1   datetime 2015 06 30      12 assert monthdiff datetime 2015 1 10   datetime 2015 01 20      0 35 assert monthdiff datetime 2015 1 10   datetime 2015 02 20      0 71   0 71 assert monthdiff datetime 2015 1 31   datetime 2015 02 01      round 1 0 31 0 2    round 1 0 28 0 2  assert monthdiff datetime 2013 1 31   datetime 2015 02 01      12 2   round 1 0 31 0 2    round 1 0 28 0 2    provides an example that works out the number of months between two dates inclusively  including the fraction of each month that the date is in  This means that you can work out how many months is between 2015-01-20 and 2015-02-14  where the fraction of the date in the month of January is determined by the number of days in January  or equally taking into account that the number of days in February can change form year to year   For my reference  this code is also on github - https   gist github com andrewyager 6b9284a4f1cdb1779b10

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This worked for me -  from datetime import datetime from dateutil import relativedelta date1   datetime strptime  2011-08-15 12 00 00     Y- m- d  H  M  S   date2   datetime strptime  2012-02-15     Y- m- d   r   relativedelta relativedelta date2  date1  r months    12 r years

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Define a  month  as 1 12 year  then do this    def month diff d1  d2           Return the number of months between d1 and d2       such that d2   month diff d1  d2     d1             diff    12   d1 year   d1 month  -  12   d2 year   d2 month      return diff   You might try to define a month as  a period of either 29  28  30 or 31 days  depending on the year    But you you do that  you have an additional problem to solve    While it s usually clear that June 15th   1 month should be July 15th  it s not usually not clear if January 30th   1 month is in February or March  In the latter case  you may be compelled to compute the date as February 30th  then  correct  it to March 2nd  But when you do that  you ll find that March 2nd - 1 month is clearly February 2nd  Ergo  reductio ad absurdum  this operation is not well defined

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Try something like this  It presently includes the month if both dates happen to be in the same month   from datetime import datetime timedelta  def months between start end       months          cursor   start      while cursor  lt   end          if cursor month not in months              months append cursor month          cursor    timedelta weeks 1       return months   Output looks like    gt  gt  gt  start   datetime now   - timedelta days 120   gt  gt  gt  end   datetime now    gt  gt  gt  months between start end   6  7  8  9  10

[python] Best way to find the months between two dates

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