Time lock

A time-lock is a mechanism whereby an event or series of events is rendered unreachable by time travel.

It is known that after the Last Great Time War, the entire war was time-locked (although it is not made clear whether the lock was in place during the war itself). Davros suggested that emperors (possibly Dalek Emperors) and Time Lords had attempted to breach a time-lock, but failed. Dalek Caan was able to overcome the lock after making an emergency temporal shift, and travelled back to the Gate of Elysium to save Davros. The process drove Caan insane, but also seemed to grant him precognitive abilities. (DW: The Stolen Earth)