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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def calculate_month(birth_date):
 """ Calculates and returns a month number basing on PESEL standard. """

    year = int(birth_date.strftime('%Y'))
    month = int(birth_date.strftime('%m')) + ((int(year / 100) - 14) % 5) * 20

    return month