Add minutes to a date and subtract minutes from a date without using any Python library

date_challenge.py

import sys
def sum_minutes_to_date(str_date, minutes):
    date, hour = str_date.split(' ')
    day, month, year = date.split('/')
    day = int(day)
    month_start_with_0 = False

    if month.startswith('0'): month_start_with_0 =  True

    month = int(month)
    year = int(year)
    h, m = map(int, hour.split(':'))
    minute = m + minutes

    if minute >= 60:
        if len(str(m)) == 1: m = '0{0}'.format(m)
        h += 1
        if h >= 24:
           day += 1
           h = '00'
    else:
        m = minute
    if month_start_with_0:
        month = '0{0}'.format(month)

    return "%s/%s/%s %s:%s" % (day, month, year, h, m)

def subtract_minutes_from_date(str_date, minutes):
    date, hour = str_date.split(' ')
    day, month, year = date.split('/')
    day = int(day)
    month_start_with_0 = False

    if month.startswith('0'): month_start_with_0 =  True

    month = int(month)
    year = int(year)
    h, m = map(int, hour.split(':'))
    if minutes >= 60:
        h -= minutes/60
        if len(str(m)) == 1: m = '0{0}'.format(m)
        if h < 1:
           day -= 1
           h = '00'
    else:
        m -= minutes
    if month_start_with_0:
        month = '0{0}'.format(month)

    return "%s/%s/%s %s:%s" % (day, month, year, h, m)


def add_or_subtract_minutes_date(str_date, operation, minutes):
    if operation in ('+', '-'):
        if operation == '+':
            return sum_minutes_to_date(str_date, minutes)
        elif operation == '-':
            return subtract_minutes_from_date(str_date, minutes)
    else:
        return'Invalid operation: {0}'.format(operation)


print add_or_subtract_minutes_date(sys.argv[1], sys.argv[2], int(sys.argv[3]))