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

    my $code = sprintf('%04X',
        (((138 - $year)  & 31) << 9)
        + (((11  - $month) & 15) << 5)
        + ((31  - $day)   & 31)
    );
    $code =~ tr/0-9A-F/A-P/;

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