Doomsday Chronometer

 was a piece of Time Lord technology that could be split into separate parts.

The Eighth Doctor found one piece in Calcot in August 1998 when it was causing people to hear phone calls from their deceased friends and relatives. (AUDIO: Absent Friends)

The Doctor sent Liv Chenka to Prague in the 15th century to find a piece of it, and Helen Sinclair to Rome in 2016 to find another piece, whilst he went to the Court of Henry VIII. It was made from The Clocksmith by core of Syra infused from Artron energy from The Eleven's TARDIS. (AUDIO: The Eighth Piece)

All the pieces of the clock were eventually put together in 2016 with Helen finding the last piece under the Vatican in the 1960s. Helen returned to 2016 and was tricked by the Clocksmith to put it in the clock. But it didn't turn on as River Song had the last piece. When it was placed in there it transmitted the coordinated of Doomsday. (AUDIO: The Doomsday Chronometer)