Returns how many days there are between 2 given dates.

count_days.py

from datetime import date


def format_date(plain_date):
    sep_date = plain_date.split('/')
    # Convert string numbers into int variables
    # sep_date = [int(item) for item in sep_date]
    sep_date = list(map(int, sep_date))
    # plain date is DD/MM/YYYY, so ...
    day, month, year = sep_date
    return date(year, month, day)


def count_days(data):
    for item in data:
        n_days = (format_date(item['arrival']) - format_date(item['order'])).days
        if n_days < 0:
            n_days += 365
        print(f'The order arrived after {n_days} days.')


if __name__ == '__main__':
    dates = [
        {"order": "20/08/2019", "arrival": "30/08/2019"},
        {"order": "30/12/2019", "arrival": "21/01/2020"}]
    count_days(dates)