Kazran Sardick's futuristic diary had been given to him by his abusive father, Elliot Sardick, in order to help Kazran become more efficient with keeping track of important times and dates. The diary had many functions. It could become a torch, and Kazran was also able to link it to the wrist watch that the Eleventh Doctor had given to him. Due to the fear he held of his father, Kazran kept the diary with him wherever he went.
When Kazran and Winston Churchill came face to face with a Dalek, they were forced to power the Dalek's ship. Although neither of them had any experience, the Dalek suggested that the diary had an efficient power source. Kazran reluctantly powered the Dalek's ship using the diary but was horrified to discover that the Dalek planned to use the ship's weapons systems to kill all life in one-hundred square miles.
As the Dalek prepared to launch its ship, Kazran noticed that his watch was counting down and remembered that the diary was still connected to the watch. Using his watch, Kazran was able to overload the Dalek's ship, causing it to explode.
Later, Winston asked Kazran what he'd tell his father about losing the gift. Kazran decided that he'd just get another. (AUDIO: Living History)