The yearometer, or year meter, (COMIC: The Doctor Strikes Back) was an instrument in the Doctor's TARDIS that showed the TARDIS' position in time and space. It stopped calculating properly after the First Doctor left England in November 1963. (TV: "The Cave of Skulls") According to another account, the yearometer had been damaged on a previous expedition. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)
Reading the year meter, the Second Doctor confirms that the TARDIS has landed in 2135 AD (COMIC: The Doctor Strikes Back)
When the TARDIS arrived at Byzantium, the yearometer gave the date as 14 March 64. (PROSE: Byzantium!)
When the TARDIS arrived at Marpling, the yearometer gave the date as 27 August 1933, though Anji Kapoor was incredulous that it was named this. (PROSE: Eater of Wasps)