sub day_dir {
    my($day, $month, $year) = (localtime(shift || time))[3,4,5];

    my @sym = ('1'..'6', 'A'..'Z');

    my $code = $sym[(138 - $year)  & 31]
             . $sym[(11  - $month) & 15]
             . $sym[(31  - $day)   & 31];

    return sprintf('%s-%04u-%02u-%02u', $code, $year + 1900, $month + 1, $day);
}