public void removeMemberCandidateDate(SourceData source, Date candidateDate) {
    LineTalkRoomConfig config = getOrCreateLineTalkRoomConfig(source.getSourceId());
    SortedSet<Date> member_candidate_dates =
        config.member_candidate_dates.computeIfAbsent(source.getUserId(), k -> new TreeSet<>());
    member_candidate_dates.remove(candidateDate);