As you may have heard, all the action of Ulysses takes place over the course of a day. Joyce kept a detailed schema that had each episode in the book beginning at a particular hour. When the bells of different churches sound in Dublin, they fit in almost perfectly with his plan. Yet time isn't just measured by the clock in Ulysses. Because we get a window into the character's minds, we also have to think about how time works in thought versus how it works in the external world. In one episode, for example, a character's dreams go on and on for what would seem like hours, and yet we find that all the external action is taking place in just a few minutes. One of the many questions posed by the book is: to what extent do we live our lives in time?
Stephen spends so much time contemplating the past because, young as he is, the future seems almost boundless. Bloom, on the other hand, mainly studies what he thinks will be useful to him because his sense of the future is limited.
Time measured by the clock and the subjective experience of time compete in the novel for the most accurate depiction of how time works. In Molly's timeless final monologue, the clock is revealed to be of only secondary importance in terms of how people think of time.