Skaro's time lock

Skaro's history was protected from potential Time Lord incursions by a time lock during the Last Great Time War. (AUDIO: Legion of the Lost) Evidently, the time lock was no longer in effect following the war as the Twelfth Doctor unintentionally arrived on Skaro during the Thousand Year War, leading to him meeting Davros as a child. (TV: The Magician's Apprentice)