Epoch of Mass Time Travel

The Epoch of Mass Time Travel, or EMTT, was the time frame in which many civilizations developed and utilized time travel in the universe. The Doctor had almost never traveled outside of this frame; indeed, very few individuals had traveled beyond the EMTT, into the extreme end of history. When the Sild brought almost all incarnations of the Master to their ship, the Third Doctor commented that the ability to reach into the EMTT meant the Sild's reach was infinite. (PROSE: Harvest of Time)