Calculate months elapsed between two dates in Python

Here is an algorithm to calculate the number of elapsed months separating two dates. It accounts for the day of month and the fact that months have different lengths. It does not matter whether days and months are zero indexed.

Algorithm

  1. Take the difference between the month numbers of date2 and date1;
  2. Add the difference between the years of date2 and date1, times 12;
  3. If the day of date2 is the last day of its month, then hold date2's day to be equal to 31;
  4. If the day of date1 is larger than that of date2, then substract 1;

Python implementation

date1 and date2 are datetime.date objects.

import calendar

def calculate_monthdelta(date1, date2):
    def is_last_day_of_the_month(date):
        days_in_month = calendar.monthrange(date.year, date.month)[1]
        return date.day == days_in_month
    imaginary_day_2 = 31 if is_last_day_of_the_month(date2) else date2.day
    monthdelta = (
        (date2.month - date1.month) +
        (date2.year - date1.year) * 12 +
        (-1 if date1.day > imaginary_day_2 else 0)
        )
    return monthdelta
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Alexandre de Verteuil
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