Timesculpting is a skill that can be used in several different ways.
Timesculpting can be used to perceive that Time has been changed in some way; the Investigator may get a feeling that some historical event "shouldn't have happened that way".
Timesculpting can be used to perceive the movement of creatures through time, such as the Hounds of Tindalos. This could be either their approach, or their departure. Consider it analogous to the movements in water, both the ripples caused by the slow approach of something, and the wake it leaves behind.
Timesculpting can be used to change chance events as well. Without magical devices or spells, the cost is prohibitive, however. The further away the event is, in time and space, the more it costs. The significance of the event, in terms of how much of the world it affects, also increases the cost.
Starting Percentage 0%
The cost in magic points to change an event in the past costs one point per second in the past, multiplied by the number of meters away, multiplied by the number of sentient beings that are aware of the event. This may be modified at the Keeper's discretion, particularly if the change will affect many beings. This is exacerbated if the beings have power over time; timescupting in the presence of Yog-Sothoth would give the Keeper the right to change the whole campaign in any way he wants!