If you are using Silverlight 3.0, then using CollectionViewSource is the cleanest way. Refer below example: (it can be done via xaml as well)
ObservableCollection<DateTime> ecAll = new ObservableCollection<DateTime>();
CollectionViewSource sortedcvs = new CollectionViewSource();
sortedcvs.SortDescriptions.Add(new System.ComponentModel.SortDescription("Date",
System.ComponentModel.ListSortDirection.Ascending));
sortedcvs.Source = ecAll;
ListBoxContainer.DataContext = sortedcvs;
And in corresponding xaml set
ItemsSource="{Binding}"
for the ListBox or any ItemsControl derived control
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