Next occurance for crontab periodic tasks

schedule = a_periodic_task.run_every
next = next_ocurrance(schedule.minute, schedule.hour, schedule.day_of_week)


def next_ocurrance(minutes, hours, days_of_week):
    # days_of_week convention: Sunday = 0, Saturday = 6
    # dateutil convention: Monday = 0, Sunday = 6

    now = datetime.datetime.now()
    weekday = now.isoweekday()
    execute_this_hour = weekday in days_of_week \
                        and now.hour in hours \
                        and now.minute < max(minutes)

    if execute_this_hour:
        next_minute = min([minute for minute in minutes
                           if minute > now.minute])
        return now + dr.relativedelta(minute=next_minute,
                                      second=0,
                                      microsecond=0)
    else:
        next_minute = min(minutes)

    execute_today = weekday in days_of_week \
                    and (now.hour < max(hours) or execute_this_hour)

    if execute_today:
        next_hour = min([hour for hour in hours if hour > now.hour])
        return now + dr.relativedelta(hour=next_hour,
                                      minute=next_minute,
                                      second=0,
                                      microsecond=0)
    else:
        next_hour = min(hours)
        next_day = min([day for day in days_of_week if day > weekday] \
                       or days_of_week)

        return now + dr.relativedelta(weekday=(next_day - 1) % 7,
                                      hour=next_hour,
                                      minute=next_minute,
                                      second=0,
                                      microsecond=0)