Sometimes it helps to read the .NET documentation even for a feature you think you know well. From WPF I knew about the CompositionTarget.Rendering event and I've also used it in Silverlight. The event is fired at the rate of video refresh and hence is ideal for performing animations. But just a couple days ago I re-read the documentation for the Silverlight version of the CompositionTarget.Rendering event and happened upon the following:
Rendering uses the EventHandler delegate and EventArgs event data by its strict signature. However, you can base a handler on EventHandler and then cast the event data to RenderingEventArgs. Then you can get the RenderingTime property value for that occurrence of the event.
RenderingTime is of type TimeSpan and indicates the elapsed time since the application began running. Using these TimeSpan values to pace animations is probably better than assuming that successive firings of the Rendering event occur at exactly the video refresh rate. Because the Rendering event handler is called in the UI thread, it is likely not precisely regular.
This casting technique also works in WPF although I haven't been able to find documentation there. (And I have a nagging suspicion that I actually knew about this at one time.) It also works in Silverlight for Windows Phone 7.
Here's a Silverlight clock that is updated from the RenderingTime property in a CompositionTarget.Rendering handler:
The clock begins when the Silverlight application begins. You can reset it by refreshing the page.
And here's the source code. The visuals are based on the notorious all-XAML clock in Chapter 30 of Applications = Code + Markup.