sub detect_dst_clockface_sector {

    my $t1 = Mktime(@_, 0, 0, 0);
    my $t2 = Mktime(Add_Delta_Days(@_, 1), 0, 0, 0);
    my $shift = 24 - ($t2 - $t1) / 3600;
    return if !$shift;

    my ($i, $it1, $it2) = 0;
    while ( $i < 24 ) {
        $it2 = $t1 + $i * 3600;
        $it1 = $it2 - 1;
        $_ = (localtime($_))[2] for $it1, $it2;
        last if $it2 == ($it1+$shift+1) % 24;
    }
    continue { $i++; }
    if ( $i < 24 ) {
        return $i, $shift;
    }
    else {
        die "Couldn't figure out position of $shift dst-affected hours";
    }
}