Time is a central motif in The Night Circus, made all the more complex for its unpredictability. The laws of physics and space are bent regularly throughout the novel, while time is a linear constraint; and yet the conception of time itself is challenged. The structure of the narrative is the easiest example: the story jumps around in time, beginning in the modern day before switching to the nineteenth century. Even smaller jumps in time contribute to the sense that time does not follow the usual rules, and the story ends where it begins, creating a sense of circularity.
Time largely adheres to typical constraints. The exception to that is aging: the inhabitants of the circus age so slowly that they appear not to age at all.
Time itself does not appear to be fated, but fate does depend on time. For example, Isobel tells Marco that she only met him because of a sudden change to her schedule, and that they were not fated to meet: it was a random accident of time. In this conversation, time is established to be something of an independent variable.
While time may be independent, it is highly unreliable. Hector Bowen highlights this to his daughter in a conversation about Celia's mother. While he only spent a few weeks in her company, he tells her, he remembers her better than Celia, who spent years with her.
Time is constantly shifted and manipulated by the story structure; but it continues to be a linear, unmoved force.