Starting "Track To" at a certain frame, not from the beginning?

is that possible? For Follow Path, I searched Google and found a way by modifying the graph parameter (not very intuitive), but how about Track to? What if I want to start it from, say frame 30?

Then you set the influence of the constraint to 0 at frame 29 and keyframe influence, then set the influence of the constraint to 1.0 at frame 30 and keyframe.