# Make a list of tuples: (index, date)
my @list = map { [ $_, $dates[$_] ] } (0..$#dates);

# Sort the list by date
@list = sort { Date_Cmp($a->[1], $b->[1]) } @list;

# Determine the corresponding ordinal numbers
my @ordinals;
foreach (0..$#dates) {
    $ordinals[ $list[$_]->[0] ] = $_ + 1;
}