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A Time-Lock was a mechanism whereby an event or series of events was rendered unreachable by time travel. The Time-Lock only prevented entry into the Time War through the time vortex itself, meaning that the Time War's events were not erased from history. They could still be referenced to by those who knew of the destruction of the Daleks and the Time Lords. The Doctor, however, would only be able to reach the Time War through time travel. Any entry into the events of the Time War would be considered a change in the time line, made impossible by the time-lock.
Use
By Time Lords
When the Doctor's TARDIS and The Master's TARDIS had each landed inside of the other, the two Time Lord's each put a time lock on the other's TARDIS to prevent escape.(DW: The Time Monster)
When the Doctor used the Moment at the end of Last Great Time War, the entire war was time-locked. (IDW: Don't Step on the Grass) Davros suggested once that Emperors and Time Lords had attempted to breach a time-lock, but failed. Dalek Caan was able to overcome the lock on the Time War after making an emergency temporal shift, and travelling back to the Gates of Elysium to save Davros. The process flooded Caan's mind with total knowledge of the past, present and future, driving him insane. (DW: The Stolen Earth) The only other things capable of breaching the time lock were those that were already in place, like the signal that had been transmitted back through time into the Master's mind, and, on at least one occasion, very small objects, such as the White-Point Star diamond sent by Rassilon to follow the signal in the Master's head. (DW: The End of Time)
The Doctor's TARDIS possessed a Time lock; its exact function is unknown. (VG: TARDIS)
By Torchwood
Another sort of time-lock, created by Torchwood 3's resident genius Toshiko Sato, was designed to freeze the Torchwood 3 Hub in a bubble in time, which was at the same time impenetrable but inescapable. (DW: Journey's End) The time lock apparently shut down when the Dalek trapped inside was destroyed as it failed to protect the hub during the events of the 456. It is possible that that time lock was destroyed when the hub blew up. (TW: Children of Earth: Day One)