you should not be using an `ArrayList` at all. You should use a strongly typed generic `List<DateTime>` instead.
For custom sorting there is an overload of the `Sort` method that takes a comparer. By reversing the regular comparison you get a sort in descending order:
list.Sort(delegate(DateTime x, DateTime y){ return y.CompareTo(x); });
###Update:
With lambda expressions in C# 3, the delegate is easier to create:
list.Sort((x, y) => y.CompareTo(x));

First of all, unless you are stuck with using framework 1.1, you should not be using an `ArrayList` at all. You should use a strongly typed generic `List<DateTime>` instead.
For custom sorting there is an overload of the `Sort` method that takes a comparer. By reversing the regular comparison you get a sort in descending order:
list.Sort(delegate(DateTime x, DateTime y){ return y.CompareTo(x); });
###Update:
With lambda expressions in C# 3, the delegate is easier to create:
list.Sort((x, y) => y.CompareTo(x));