The 2000 Year Diary was a journal kept by the Twelfth Doctor.
It was an updated version of his 1200 Year Diary, (TV: The History of the Doctor) which was, in turn, a continuation of his Nine Hundred Year Diary, (TV: Doctor Who) which itself was a follow-up on his Five Hundred Year Diary. (TV: The Power of the Daleks, etc.)
The Doctor used the Two Thousand Year Diary to remind himself of the Mire after they threatened a Viking village. (TV: The Girl Who Died)
At some point the diary would be replaced by the Five Thousand Year Diary. (GAME: Lost in Time)
Other realities[]
In the Daft Dimension, the Twelfth Doctor was already lugging around a Two Thousand Year Diary shortly after his regeneration. This version, owing to all the information it had to contain, was larger than the Doctor himself, and nearly impossible for him to carry around. Clara Oswald tried and failed to convince him to convert it to a more easily browsed digital format. (COMIC: The Daft Dimension 478)