@Override
    public int addFutureMeeting(Set<Contact> contacts, Calendar date) {
        if(date.before(timeNow()) || !contactsExist(contacts)) {
            throw new IllegalArgumentException();
        } else {
            int id = generateMeetingId();
            allMeetings.add(new FutureMeetingImpl(id, date, contacts));
            return id;
        } 
    }