Gate of the Past or Future

The Gate of the Past or Future, known to the Pythias as the Gate of the Future and known to the Time Lords as the Gate of the Past, was a barrier which separated the Old Time of the Pythias from the New Time of the Time Lords. It existed in the astral plane. Time flowed from the future to the past under this Gate, such that from the Future's perspective the Gate swallowed time into the Past and from the Past's perspective time flowed from the Future. (PROSE: Lungbarrow)

The Pythias often accessed the Gate using fumes from the Crevasse of Memories That Will Be. (PROSE: Cat's Cradle: Time's Crucible)

Rassilon would claim to have "shut the gate on the Time of Chaos." (PROSE: Lungbarrow)

Related sources would concur that there was a powerful barrier blocking time travel into the Dark Times. (PROSE:  Revelation, No Future, Goth Opera)