Techniques and tools described herein provide effective ways to program a property of a target object to vary depending on a source. For example, for a key frame animation for a property of a target UI element, an alternate time source is set to a property of a source UI element. When the target UI element is rendered at runtime, the animation changes the target value depending on the value of the property of the source UI element. Features of UI elements and animations can be specified in markup language. The alternate time source can be specified through a call to a programming interface. Animations for multiple target UI elements can have the same source, in which case different parameters for the respective animations can be used to adjust source values in different ways.