A diary was a record made by an individual of events that occurred in their life.
The Doctor had a Five Hundred Year Diary. (PROSE: The Three Paths, TV: The Power of the Daleks) He later updated this to a Nine Hundred Year Diary, (TV: Doctor Who) and later still to a Two Thousand Year Diary. (TV: The Girl Who Died)
Richard Haliwell's diary was placed in a time safe in the Doctor's TARDIS so he would have it earlier to use for reference. (PROSE: Imperial Moon)
Each time the Doctor and River Song met, she would compare her diary with his to see where they were in their relative time streams. (TV: The Impossible Astronaut, TV: Let's Kill Hitler)
Professor Frederick Marius kept a computer diary at least during the time he acquired K9 Mark III. (PROSE: Tautology)