SortedList Desc Order

You can just use Reverse() to sort the SortedList in descending order:

var list = new SortedList<DateTime, string>();

list.Add(new DateTime(2000, 1, 2), "Third");
list.Add(new DateTime(2001, 1, 1), "Second");
list.Add(new DateTime(2010, 1, 1), "FIRST!");
list.Add(new DateTime(2000, 1, 1), "Last...");

var desc = list.Reverse();

foreach (var item in desc)
{
    Console.WriteLine(item);
}

Swapping y for x should do when comparing

class DescComparer<T> : IComparer<T>
{
    public int Compare(T x, T y)
    {
        if(x == null) return -1;
        if(y == null) return 1;
        return Comparer<T>.Default.Compare(y, x);
    }
}

and then this

var list = new SortedList<DateTime, string>(new DescComparer<DateTime>());

There is no way to instruct the SortedList to do sorting in descended order. You have to provide your own Comparer like this

    class DescendedDateComparer : IComparer<DateTime>
    {
        public int Compare(DateTime x, DateTime y)
        {
            // use the default comparer to do the original comparison for datetimes
            int ascendingResult = Comparer<DateTime>.Default.Compare(x, y);

            // turn the result around
            return 0 - ascendingResult;
        }
    }

    static void Main(string[] args)
    {
        SortedList<DateTime, string> test = new SortedList<DateTime, string>(new DescendedDateComparer());
    }

Comparer<DateTime>.Create((x, y) => 0 - Comparer<DateTime>.Default.Compare(x, y));