There isn't a time lock on Skaro. The time lock is on the Last Great Time War. The Doctor can visit the planet, as long as he does so outside the war.
Note that the same does not apply to Gallifrey. Gallifrey before the Time War is protected against anyone from its future using time travel to reach it. Gallifrey during the Time War is time locked because the war is time locked. Gallifrey after the Time War doesn't exist in this universe.
- ↑ He met Darla Von Karlsen on the planet in "Asylum of the Daleks".
- ↑ This didn't prevent an "echo" of Clara Oswald being on Gallifrey when the First Doctor stole the TARDIS but, according to Clara in "The Name of the Doctor", that "echo" didn't get there by time travel but by being born there.
- ↑ As we learned in "The Day of the Doctor", Gallifrey does still exist in "a frozen moment of time" in a pocket universe of its own.