A Timeline, by itself, doesn't actually do anything other than describe a segment of time. It's the timeline's Clock object that does the real work: it maintains timing-related run-time state for the timeline.
In most cases, a clock is created automatically for your timeline. When you animate by using a Storyboard or the BeginAnimation method, clocks are automatically created for your timelines and animations and applied to their targeted properties. For examples, see How to: Animate a Property by Using a Storyboard and How to: Animate a Property Without Using a Storyboard.
You can also create a Clock explicitly by using the CreateClock method. In performance-intensive scenarios, such as animating large numbers of similar objects, managing your own Clock use can provide performance benefits.
Clocks are arranged in trees that match the structure of the Timeline objects tree from which they are created. The root clock of such a timing tree can be interactively manipulated (paused, resumed, stopped, and so on) by retrieving its Controller. Non-root clocks cannot be directly controlled.
Once created, a clock cannot be modified (but it can be manipulated).
Using a Timeline as a Timer
A timeline's clock will only progress when there's an event handler associated with it or (in the case of an AnimationClock object) it is associated with a property. For this reason (and others), it's not recommended that you use a Timeline as a timer.
Derived classes should implement GetCurrentTimeCore if they want to modify how time flows for this clock. Derived classes can be made to do additional work when the clock repeats, skips, seeks, begins, pauses, resumes, or stops by overriding the DiscontinuousTimeMovement, SpeedChanged, and Stopped methods.