Lodestone

The Lodestones were a type of time travel technology used by the Osirian civilisation.

They took the form of an Egyptian sarcophagus and were entrances into a time space tunnel. When activated, the coffin shaped device loses its stone form and becomes a visible doorway with energy spiralling in its form.

One such lodestone threw the Fourth Doctor's TARDIS off course. (TV: Pyramids of Mars)

Lodestones differed from most forms of time travel in that there was a 1:1 ratio between the distance in time travelled and the time experienced for the traveler; for example, a person on a journey 70 years back in time through a lodestone would age 70 years. As the Osirans lived thousands of years, this was not a concern to them. (PROSE: The Sands of Time)