$data{$_->[0]}{$_->[1]}{$_->[2]} = '' for map [split], <DATA>;

for my $num (reverse sort keys %data) {
    print "$num\t$_\t", (join ",", keys %{$data{$num}{$_}}), "\n"
        for keys %{$data{$num}};
}

__DATA__
848	05/23/06	11:00
848	05/23/06	12:30
848	05/23/06	13:00
848	05/23/06	14:00
848	05/25/06	11:00
848	05/25/06	12:00
261	05/24/06	11:00
261	05/24/06	12:30
261	05/24/06	13:00
261	05/24/06	13:00
261	05/24/06	13:00
261	05/24/06	13:00