Calculate the fractional months between two dates, inclusively.

monthfraction.py

from datetime import datetime, date
import calendar

def monthdiff(start_period, end_period, decimal_places = 2):
	if start_period > 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 > 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 less 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)

Why don't you provide your example of dates.

If your dates were 2020-04-01 to 2020-05-03, you would have 1 month (2020-04-01 to midnight 2020-04-30) plus then 3 of the 31 days of may, or 3/31 or 0.096 (or 0.10) of the month of may. That is the portion of the month of May that is included in your calculation.

The other way to view the part of the month is to think about it this way: the month of May has 31 days. So 1 day in the month of may is 1/31. Three days, is therefore 1/31 * 3, or 3/31.

Andrew

I wanna use this to calculate the number of months between each month of the dataset and the current months.
Example 👍
today = date.today()
df_req1["ancienté_par_rapport_year"]=today.year-df_req1["year"]
if I have "2006-06-01" and today I know it's 14 years
I want the same thing for months and days