Calculates and returns a month number basing on PESEL standard.

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