500 Year Diary

The Five Hundred Year Diary was a journal kept by the Doctor.

After regenerating, the Doctor got his diary out of his storage chest and began to read it, even while walking through the mercury swamps of Vulcan. (DW: The Power of the Daleks)

While on Earth, the Doctor checked his pockets for his diary, which contained notes he made about the Sontarans. (DW: The Sontaran Experiment)

While on a planet made of confectionary, the Doctor checked the calendar in his diary and learned that it was K-9's birthday. (ST: Special Occasions: 1. The Not-So-Sinister Sponge)