Diaries are a dizzying technology. They are a subject equally worthy of a sonnet and a philosophical treatise. Even the most humble daily record book, which might easily be mistaken for any other humdrum accessory, is a hand-held time machine. Electronic diaries are not as romantic as their physical counterparts, but whether swiping screens or turning real pages, these temporal teleporters work in the same way, and their power is astonishing. Moving left looks into the past, nudging recollection: moving right propels us into what has yet to come. Gwendolen has that lovely line in The Importance of Being Earnest about always travelling with her diary because “One should always have something sensational to read in the train.” Her witticism is not actually prompted by a desire to get lost in racy reminiscence; on this occasion, she simply wants to establish a point of historical fact. Yet the fact she is able to demonstrate – that she received a proposal of marriage “yesterday afternoon at 5.30” – itself proves sensational. Diary entries do not, then, need to be extensive in length or exotic in theme to cause a stir; the scantiest notes will do. “Oh, I’d forgotten all about that!”; “Was it really so long ago?”; “I cannot wait another whole week”: scanning back and forth between what we have done and what awaits us down the track can be a highly charged and vertiginous business. Perhaps most arresting of all are not the individual moments that diaries record so much as the way, within a diary, separate entries begin to form patterns. If Aristotle was right about character being formed through habits of behaviour, our diaries may tell a chastening story not only about what we have done but also about who we are, and who we might become.